Lee A Groat. American Scientist. Volume 100, Issue 2. Mar/Apr 2012.
The sparkle and luster of gemstones has made them prized objects for thousands of years. Gems are valued for their color, luster, transparency, durability and high value-to-volume ratio. Because many gems are produced from relatively small, low-cost operations in remote regions of developing countries, it is difficult to obtain accurate statistics regarding their production and value. However, world production of uncut diamonds was worth $12.7 billion in 2008, and in 2001 the trade journal Colored Stone calculated that the world coloredgem trade was worth about $6 billion per year. Although synthetic forms of many gems now exist, they have yet to have a serious impact on the international gemstone market.
Part of the reason that gemstones reach such high values is their rarity. A typical diamond deposit yields 5 grams of gems per million grams of mined material, with only 20 percent of the gems being of jewelry quality. Like oil, gems can take an immense stretch of geologic time to form. Radioactive-decay dating of microscopic inclusions in diamonds has found these gems to be 970 million to 3.2 billion years old. Thus high-quality gems can be mined out much faster than they are produced, essentially making them a finite resource. For instance, one emerald mine established in 1981 in Santa Terezinha, Brazil, produced a peak of 25 tons of rough stones valued at $9 million in 1988; the same tonnage of stones mined in 2000 sold for only $898,000. This scarcity also makes gemstones highly valuable to geologists. Exceptional geological conditions are required to produce gem deposits. The desire to unravel the history of such unusual circumstances is drawing increasing numbers of Earth scientists to the study of gems and their origins.
Although there are dozens of different types of gems, among the best known and most important are diamond, ruby and sapphire, emerald and other gem forms of the mineral beryl, chrysoberyl, tanzanite, tsavorite, topaz and jade. (Common gem materials not addressed in this article include amber, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)
Diamond is the crystalline phase of carbon formed at very high pressures. It is the most highly valued gem; exceptional stones can fetch upward of $500,000 per carat (1 carat = 0.2 grams) and individual pieces can be valued at more than $20 million.
The Golkonda region in south-central India was the original source of diamonds for hundreds of years, until discoveries were made in Brazil during the 18th century and at Kimberley, South Africa, in 1866. Today, the top three diamond-producing nations by value are Botswana, Russia and Canada, with significant production from Angola, Australia, Congo, Lesotho, Namibia, Sierra Leone and South Africa.
Diamond crystallizes in the cubic system, meaning that its constituent carbon atoms are arranged in cells with axes of the same length (specifically 0.356 nanometers) at right angles to each other. This formation results in a number of shapes, including octahedra, cubes, cubo-octahedra and less regular aggregates. Diamond can exhibit a number of distinct physical properties, such as emitting a glow under ultraviolet (UV) light or x-rays. Such x-ray fluorescence is exploited when processing ore to distinguish diamonds from the waste rock. Gems emit visible light when hit with UV or x-rays because defects in the crystal structure absorb the radiation, causing their constituent electrons to vibrate between energy levels, and the energy is released as light. (Gemstones that have been treated with heat or radiation, or are synthetic or fakes, will often fluoresce at different wavelengths, so this property can also be used to verify real stones.)
Diamonds are divided into types according to the presence or absence of nitrogen and boron, as well as the structural organization of these impurities within the crystal lattice. Type I diamonds are described as containing significant nitrogen that is detectable by infrared absorption spectroscopy (a process that detects which wavelengths are absorbed or transmitted by a stone, each element being associated with typical wavelengths). Type II diamonds do not contain significant nitrogen.
Color in natural diamond is related primarily to the substitution of other elements for nitrogen and other defects caused by physical deformation in the crystal lattice; there are often multiple color-causing defects in a single sample. Type I diamonds in which the nitrogen impurities are clustered are generally colorless, brown or yellow; when the impurities are more widely diffused the diamonds are yellow, orange or brown. Pink, red and purple diamonds are also of Type I, and the coloration has been tied to deformation of the impurity-laden part of the crystal lattice after the gem finished forming. Type II diamonds contain very few or no nitrogen impurities but may have boron impurities, which typically render the diamond blue to gray. When they are virtually devoid of all impurities, they are colorless or brown.
Mineral inclusions within diamonds permit calculation of pressures and temperatures of the environment in which they formed. Diamonds generally crystallize at depths of 135 to 200 kilometers and at temperatures of 1,100 to 1,200 degrees Celsius. The vast majority originates from within the lithosphere (the rigid crust and upper mantle of the Earth) below very old, stable parts of the continental crust called cratons—areas toward the center of tectonic plates that are far from areas of growth or subduction. The rest originate largely from sublithospheric sources, which can be as deep as the lower mantle. Such sources are generally described as being within deep keels of ancient cratons, where geothermal energy is suppressed by these relatively cold masses, thus allowing crystallization to occur. This setting is low in silica, and is dominated by rocks such as peridotite or eclogite, both of which are high in magnesium and iron.
Well-formed diamond crystals most likely result from two processes. One is the reduction (the gain of electrons) of oxidized carbonate (CO3) in its solid state, or dissolved within a melted rock or chemical-rich fluid. The other is the oxidation (the loss of electrons) of reduced carbon in the form of methane. Crystallization in either a molten-rock or a fluid-dominated setting allows physically unconstrained crystal growth.
Carbonate that is reduced to diamond would likely be in the form of the minerals dolomite [CaMg(COsJa] or magnesite (MgCO3). The carbonate component has been hypothesized to originate from carbon introduced into the mantle when volatile chemicals, such as CO2, escape oceanic crust as it subducts beneath another tectonic plate and enter a region of molten rock.
Methane that is oxidized to diamond is thought to originate from reduced fluids in the upper mantle. The release of water by this reaction would aid in fluiddriven, or metasomatic, reactions at the site of diamond growth in the subcratonic lithosphere. The ultimate origin of carbon in diamonds, however, is ambiguous and a subject of ongoing research.
Models for mantle evolution suggest that diamonds older than 2.5 billion years were most likely generated from methane oxidation, whereas those younger than 2.5 billion years were mainly formed through carbonate reduction.
Radiometric dating determines age by measuring the percentage of different isotopes (variants of elements that differ only in the number of neutrons) in a material that has naturally decayed over geologic time. Such analysis of diamond is achieved by studying its silicate and sulfide mineral inclusions that crystallized at the same time as the host diamond. However, analyses of minerals along exposed planes where the crystal has cleaved have been shown to record the date of the eruption that brought the diamond to the surface, not necessarily the date of its formation.
Regardless of depth of formation, diamonds are transported quickly to the surface via rapidly rising bodies of molten rock—called kimberlite or 1amproite magmas—that originate from the growth areas themselves or from greater depths. The transporting magma may be corrosive to diamond and thus requires a speedy ascent to preserve the gems. Kimberlites erupt at average speeds of 10 to 30 kilometers per hour through the rapid release of carbon dioxide and water, creating buoyancy. However, the mechanism of this release was not clear until recent work by James K. Russell and his colleagues at the University of British Columbia. In high-temperature experiments, Russell’s group showed that it is not a decrease in pressure that causes the release, but the magma’s travel upward from a region that is carbon-rich to one that is silica-laden (a property found in cratons, which could help explain diamond’s association with those regions). An increase in silica content in the magma causes a quick drop in its solubility of carbon dioxide, causing continuous and vigorous expulsion of the gas and driving the ascent of the kimberlitic magma.
Kimberlites are usually about 65 to 135 million years old, but some are as old as 1.1 billion years. They are not as aged, however, as the diamonds they end up transporting. With few exceptions, diamond formed well before kimberlite or lamproite eruption—on the order of hundreds of millions to billions of years. Although eruption ages of diamondiferous kimberlite are variable, those kimberlites younger than 1.6 billion years comprise the majority of economically significant bodies. Radiometric dating indicates that kimberlite eruptions have grown more frequent over time, with some of the youngest dated ones belonging to the Eocene (56 to 34 million years ago) kimberlite clusters of the Lac de Gras area in Canada’s Northwest Territories. Diamond deposits older than about 1.6 billion years are in the form of paleoplacers (sediments created by gravity separation that have been compacted into rock), lamprophyric dikes (sheets of low-silica rock that cut through other geological layers) and breccias (broken rock fragments that have been cemented into a solid), as well as rare diamondiferous but uneconomic kimberlite.
Besides its beauty, diamond’s exceptional physical properties, unusual formation and value have prompted abundant research. Studies on the origin of host structures and global distribution of the gemstone have facilitated research and understanding of the deep Earth and led to various methods for laboratory synthesis of diamonds.
Ruby and Sapphire
Ruby and sapphire are gem varieties of the mineral corundum, essentially an oxide of aluminum that has the general formula Al2O3. Ruby and sapphire are perhaps the world’s most widely sold colored gemstones, accounting for approximately one-third of sales by value. They can command some of the highest prices paid for any gem: In 2006 an 8.62-carat Burmese ruby sold for $3,640,000, and in 2009 a 16.65-carat Kashmir sapphire was purchased for $2,396,000.
Corundum crystallizes in the hexagonal system—the crystal’s three axes on the horizontal plane intersect at 60-degree angles, and the fourth, vertical axis intersects at 90 degrees. Ruby is red and sapphire is blue; all other colors are referred to as sapphire with a modifier (such as “yellow sapphire”). Both ruby and sapphire can exhibit asterism or “stars” on the surface of round-cut stones, called cabochons. These are caused by light reflecting from needlelike inclusions of a titanium-oxide mineral called rutile, or other iron or irontitanium oxide phases, aligned along crystallographic planes and parallel to the hexagonal faces at 60 degrees.
The color of ruby is due to chromium replacing aluminum in the crystal structure. Chromium is also responsible for the green color of emerald, and the reason for the difference in color between emeralds and rubies is still unresolved. In both gems, the chromium is surrounded by six oxygen atoms, but absorbs light differently in each crystal. One theory for the color variation is that it is caused by the electrostatic potential imposed by the rest of the lattice ions on the active electrons of the chromiumoxygen unit. The main effects are thought to be from the electric field generated in the neighborhood of the chromium-ion site in ruby, which is absent in emerald because of the symmetry of its lattice. This charge results in the absorption features being shifted to higher energies in ruby such that the gem has two large bands of visible light absorbed at wavelengths of approximately 400 and 550 nanometers, and two transmission windows at 480 nanometers (blue) and 610 nanometers (red). Ruby appears red because the human eye is more sensitive to red above 610 nanometers than to blue. Red fluorescence under ultraviolet light and sometimes daylight, combined with the red color of ruby, is the cause of the fire effect seen in many rubies from Myanmar and Vietnam.
The blue color of sapphire results from electron transfer between less than 0.01 percent of iron (Fe2+) and titanium (Ti4+) ions replacing aluminum ions (Al3+) in the crystal structure. This charge transfer uses specific amounts of energy from light at certain wavelengths; the wavelength used is absorbed and not seen. In sapphire, light from the red end of the srjectrum is used as energy for the charge transfer between iron and titanium atoms, making the gem look blue. Colorless “gueda” sapphire is commonly heated to achieve greater transparency and blue colors by melting inclusions to release iron and titanium.
Orange-pink gem corundum is called padparadscha, from the Sanskrit term for the color of the lotus flower. Aluminum in the crystal structure is replaced by a combination of chromium ions (Cr3+), which create a pink hue, and iron ions (Fe3+), which undergo a charge transfer with oxygen ions (O2”) to produce a yellow hue.
Most gem corundum is produced from placer deposits that are classified as alluvial (water transport), colluvial (gravity transport) and eluvial (weathering). Gem corundum is also produced from paleoplacers.
The global distribution of corundum deposits is linked to collision, rift and subduction geodynamics. Three main periods of corundum formation are recognized: the Pan- African orogeny (750 to 450 million years ago), which produced primary gem corundum deposits in Africa, India, Madagascar and Sri Lanka; the Himalayan orogeny (45 to 5 million years ago), which produced the marble-hosted ruby deposits in Asia; and Cenozoic alkali-basalt extrusions (65 to 1 million years ago). The traditional sources are Kashmir, Myanmar, Sri Lanka and Thailand. Newer major producers include Australia, Madagascar and Vietnam.
The finest rubies and sapphires come from thick marble layers composed of calcite (CaCO3). But how did the aluminum needed to form corundum get into the marble? How about the chromium, titanium and iron needed to impart color? Why is there no silica, which is three times more common in the Earth’s crust? If silica were present, it would bind with aluminum and prevent the formation of corundum. My colleagues and I are currently studying a ruby and pink-sapphire deposit in central British Columbia to investigate these questions. Preliminary results suggest that the material precursor to the marble was limestone deposited in thin interlayers of mudstone. When the limestone metamorphosed to form marble, the minerals in the mudstone (primarily mica, containing silica and many other trace minerals) underwent a complicated series of reactions to ultimately form corundum.
Emerald is the green gem variety of the beryllium-based mineral beryl with general formula Be3Al2Si6OIg. Like corundum, beryl crystallizes in the hexagonal system. The color of emerald is due to trace amounts of chromium, vanadium or both elements replacing aluminum in the crystal structure. In the beryl crystal, rings of silicon and oxygen are stacked, leaving channels in the center that can trap water or other impurities.
Emerald is one of the most valuable gemstones. The highest price ever paid for an emerald was $1,149,850 for a 10.11-carat Colombian stone in 2000. The pricing of emeralds is unique in the colored gemstone market because a greater importance is placed on color than on clarity, brilliance or other characteristics.
Colombia is thought to supply an estimated 60 percent of the world’s emeralds. Official production in 2001 was 5.5 million carats, worth more than $500 million. Zambia is considered to be the world’s second most important source of emeralds by value.
Beryl may contain significant amounts of water at the channel sites. When heated above 400 degrees, the trapped water breaks into gaseous molecules that are confined to the channel voids. The channel water is liberated at temperatures of about 800 degrees, without much effect on the natural ratio of hydrogen isotopes. Thus, channel water may represent the original fluid composition from the time of formation, and the measurement of the change in ratio of hydrogen isotopes in water released from beryl may permit determination of the source of the fluids from which the beryl grew.
Isotopie compositions of historical emerald artifacts have been used to show that during historical times, artisans worked with emeralds originating from deposits that were supposedly discovered only in the 20th century. Research by Gaston Giuliani and his colleagues at the Center for Petrographical and Geochemical Research in France found that most of the highquality emeralds cut in the 18th century in India were transported there from Colombia, whereas previous studies had assumed a closer origin.
Beryl and chrysoberyl (described in the following section) are relatively rare because there is very little beryllium in the upper continental crust. Beryllium tends to be concentrated in rocks such as granites, pegmatites (formed from water-rich magmas) and black shales (a sedimentary rock), as well as rocks that these materials can metamorphose into. Chromium and vanadium are more common in the upper continental crust and are concentrated in igneous rocks such as peridotites, dunites (a type of peridotite) and basalts (quickly cooled magma) of the oceanic crust and upper mantle and their metamorphic equivalents. However, high concentrations of chromium and vanadium can also occur in sedimentary rocks, particularly black shales.
Thus, unusual geologic and geochemical conditions are required for beryllium (found mostly in rocks that form within continents) to meet with chromium or vanadium (concentrated in volcanic rocks associated with ocean ridges). In the classic model, berylliumbearing pegmatites, in their magma state, interact with chromium-bearing ultramafic or mafic rocks (those high in magnesium and iron). However, researchers are recognizing that other geological events involving tectonics may play a significant role in certain emerald deposits.
A case that does not fit the classic model is the Colombian deposits, where there is no evidence of magmatic activity. The more than 200 emerald deposits and occurrences in Colombia are located in two narrow bands on both sides of the Cordillera Oriental mountain range. The Colombian emeralds formed as a result of hydrothermal sgrowth associated with tectonic activity. A number of studies have pointed to an evaporine origin for the parent hydrothermal fluids. The fluids are thought to have formed from water interacting with salt beds in black shales, which were buried to depths of at least 7 kilometers and reached temperatures of at least 250 degrees. The highly alkaline briny fluids migrated upward through the sedimentary layers along décollement thrust planes (gliding planes between two rock masses that allow independent types of deformation in the rocks above and below the fault) and then interacted with the black shales.
It is generally agreed that organic matter, and sulfur derived from it, is important in the formation of the Colombian emerald deposits, but debate exists over the exact nature of the associated reaction.
A similar origin has been described for very rare emerald mineralization in the Neoproterozoic (1 billion to 542 million years ago) Red Pine Shale of the southwestern Uinta Mountains in Utah: Sulfate-bearing brines from the Uinta Basin migrated upward along the South Flank Fault Zone and interacted with the carbon-rich shale. The sulfate was reduced by organic carbon to form sulfides and emerald.
There has been considerable debate over the role that tectonic processes play in the formation of some emerald deposits. For example, a regional-metamorphic origin has been suggested for emeralds at Habachtal in Austria. This conclusion is supported by the physical evidence of metamorphism, shearing and multiple growth stages, such as augen (eyeshaped) textures in country rocks and emeralds with curved inclusion trails. Critics of this interpretation say the classic pegmatite model could still apply. Although pegmatitic sources of beryllium may not be apparent, fluids can travel far from pegmatites, especially along intensely sheared rocks, and pegmatites do occur in the region.
Recently, boron isotopes of tourmaline coexisting with emeralds were used to study the origin of the Habachtal deposit. The boron isotope values found suggest that two separate fluids were channeled and partially mixed in a shear zone during tourmaline-emerald mineralization, but neither of these fluids is associated with pegmatites. A regional-metamorphic fluid carried isotopically light boron, as observed in the country rocks. The other fluid derived from interlayered serpentinites (mafic rocks oxidized by water) and has isotopically heavier boron that is typical for mid-ocean ridge basalts or altered oceanic crust.
The same method was applied to the Tsa da Glisza emerald occurrence in Yukon, Canada. Here, the principal occurrence of emerald is along contacts between quartz-tourmaline veins and mafic country rocks. In this case, the isotope values were reported to be consistent with a dominantly granitic source of boron, with a contribution of isotopically heavy boron from maficultramafic formations.
Other members of the beryl family that have been used as gems include aquamarine and maxixe (blue), golden beryl (yellow), heliodor (greenish yellow), goshertite (colorless), morganite (pink) and red beryl. All except red beryl are ordinarily found in pegmatites and certain metamorphic rocks.
Our group has suggested that the color of blue beryl can be attributed to charge transfer between iron ions in different oxidation states. Iron cations (Fe2+) at the aluminum sites exchange electrons with small amounts (about 0.04 atom per formula unit for darkblue material) of Fe3+ cations. Aquamarine is produced from mines in Brazil, Colombia, Kenya, Madagascar, Malawi, Russia, Tanzania and Zambia, and from placers in Sri Lanka.
Maxixe is a dark-blue gem beryl from Brazil in which the color results from the inclusion of nitrate (NO3) and carbonate (CQ3”) at the centers of the crystal ring structure. The color fades with prolonged exposure to light as a result of hydrogen atoms decaying in the crystals. A recent study has suggested that the nitrate is created by a natural process, whereas the carbonate is due to irradiation from surrounding rock.
The color of golden beryl and heliodor is attributed to iron cations (Fe3+). The pink color of morganite (named for financier J. P. Morgan) is attributed to manganese (Mn2+) ions. Morganite occurs in Afghanistan, Madagascar and at Pala in California.
The gem value of goshenite is relatively low; to increase value it can be colored by irradiation with high-energy particles or by electrolysis (splitting materials with an electric current), with the resulting color depending on the impurities. In the past, goshenite was used for manufacturing eyeglasses and lenses owing to its transparency.
The color of red beryl is due to manganese (Mn3+) ions. Red beryl has only been reported from topaz-bearing rhyolites (a type of volcanic rock high in silicon and low in iron and magnesium) in Mexico, New Mexico and Utah. The beryllium necessary to form the red beryl is thought to be derived from the host rhyolite. Mobilization of the beryllium, in the form of beryllium-fluorine complexes, was promoted by the very low calcium contents of the host rock, which inhibited the formation of fluorite (CaF2). This movement took place when fluorine-rich gases from the cooling rhyolite mixed with vapors from heated ground water to produce a supercritical fluid—its temperature and pressure were above a threshold point where it can seep through solids like a gas but still dissolve materials like a liquid. The absence of clay mineral inclusions in the red beryl suggests that it formed at temperatures below those required for the crystallization of rhyolite magma (less than 650 degrees) but above those of clay alteration (200 to 300 degrees).
The only known commercial occurrence of gem-quality red beryl is the Ruby Violet (or Red Beryl) mine in the Wah Wah Mountains of Beaver County, Utah. Total production for the 25 years prior to 2003 was more than 60,000 carats, of which about 10 percent was facetable. Prices can be as high as $10,000 per carat for faceted stones.
Based on a study of material from Poona, Australia, and Kifubu, Zambia, in 2006 Arûnas Kleismantas and his colleagues at Vilnius University in Lithuania proposed the new gem beryl variety ”chromaqiiamarine” with composition close to emerald but with considerably more iron (0.48 to 1.11 percent) than chromium (0.08 to 0.15 percent) or vanadium (less than 0.02 percent).
Another beryllium-based gem, distinct from the beryls, is chrysoberyl, which has the general formula BeAl2O4. It is generally a golden-yellow, to a greenishor brownish-yellow color. The color is due to iron cations (Fe3+) substituting for aluminum in the crystal structure. The bulk of gem chrysoberyl available in recent years has come from alluvial deposits in the Bahia, Espirito Santo and Minas Gerais states of Brazil.
The two most valued varieties of chrysoberyl are alexandrite and cymophane. Alexandrite ranges in color from greenish blue in natural light to deep red under incandescent light- The effect is from small amounts of chromium substituting for aluminum in the crystal structure. Cymophane is translucent chatoyant or “caf s eye” chrysoberyl In this variety, needlelike inclusions of rutile produce an effect visible as a bright band of light that moves across the stone as it is rotated. This effect is best seen in gemstones cut in cabochon form. Cat’s eye material is found as a small percentage of the overall chrysoberyl production.
There has been much debate about the origin of chrysoberyl deposits. Most are associated in some way with pegmatites, but in many cases the stone is associated with aluminum-rich minerals absent in most pegmatites. Some studies have concluded that under conditions of high temperature and pressure, the assemblage of beryl and duminum-silicate is unstable and decomposes to the assemblage of chrysoberyl and quartz.
Chrysoberyl was discovered in association with emerald and phenakite (another beryllium-silica mineral) at Franqueira in northwestern Spain in 1968-1969. The chrysoberyl is often cyclic twinned (nonparallel crystals share some of their crystal-lattice points), and many crystals show the alexandrite effect of color shifting under different types of light. The beryllium minerals occur in zones rich in phlogopite (a phosphate mineral) in a dunite (an ultramafic rock) intruded by a pegmatite associated with a type of granite that is high in aluminum oxide and potassium. The chrysoberyl could have formed by the buildup of thin layers on existing olivine crystals (a magnesium-iron silicate mineral), because the two minerals share features in their crystal structures.
Study of additional samples resulted in the conclusion that chrysoberyl formed from the breakdown of another mineral called sapphirine (a silicate of magnesium and aluminum with a high beryllium content, named for its color being similar to sapphire) during metamorphism after it was formed.
Textural and compositional evidence suggests that chrysoberyl formed during the regional metamorphism of granulite facies (medium or coarse-grained metamorphic rock bodies that underwent intense pressure and temperature changes), a process that could have released beryllium from the host sedimentary rock.
Tanzanite and Tsavorite
Tanzanite is the dark-blue gem variety of the mineral zoisite, with the general formula Ca2Al3(SiO4)(Si2O7)O(OH). The blue color is due to vanadium, which substitutes for aluminum in the crystal structure. Most uncut tanzanite is grayish brown, grayish purple, brownish purple, bluish and greenish brown, and must be heat treated to alter impurities and remove undesirable hues.
Tanzanite was discovered in 1967 near Merelani in northeastern Tanzania, which remains its only known source. The deposits are located in the western flank of a series of folded and metamorphically deformed rocks called the Lelatema Fold Belt. The tanzanite crystals are commonly found either in cavities at the hinges of folds in quartz veins that have been stretched by geologic processes after their formation, or bedded in hydrothermally altered gneisses (a metamorphic rock with visible layering), marbles and calcium-silicates. Mineral compositional and textural data suggest that chromium and vanadium were leached from black shale undergoing prograde metamorphism (where increasing pressure and temperature drives off volatile chemicals), and were concentrated during a retrograde hydrothermal metamorphic episode (when the rock was cooled and could reincorporate volatiles) to form tanzanite and other minerals. The tanzamte-forming fluids are estimated to have circulated along fold hinges at temperatures between 390 and 450 degrees and at a pressure of about 3 kilobars. Fission track dating (a technique that examines the damage trails left by naturally decaying materials) of tanzanite suggests a crystallization age of 585 million years ago.
The famous jewelry retailer Tiffany & Co. introduced the name of the gem in 1969. The worldwide market for rough tanzanite is estimated to be worth around $100 million per year. For extremely fine stones of less than 50 carats, prices can attain $1,000 per carat.
Tiffany & Co. also named the gem tsavorite, after Tsavo National Park in Kenya. Tsavorite is a green gem variety of a caldum-aluminum type of garnet called grossular (named after the gooseberry, Ribes grossularia, of similar color) that has the general formula Ca3Al2(SiO4J3. The green color of tsavorite is due to vanadium substituting for aluminum in the crystal structure. Faceted tsavorite gems of three carats or more are very rare.
The green-gem garnet that became known as tsavorite was discovered in 1967 in northeastern Tanzania. Most of the world’s tsavorite mines and significant deposits are in eastern Tanzania and southeast Kenya, near the Tanzanian border. All of these deposits are hosted in vanadium-rich graphitic gneisses associated with marbles in formations that are metasedimentary (sedimentary rocks that show significant signs of metamorphism).
All of the Merelani, Lemshuku and Namalulu mining districts in Tanzania are on the western flank of the Lelatema Fold Belt. The best tsavorite crystals occur in the Merelani district, where they are found with tanzanite and chromium-bearing tourmaline in hydrothermally altered graphite gneiss. As with tanzanite, the gem crystals are concentrated in quartz veins, fold hinges and stretched structures.
At the Komolo and Lemshuku mines in the Lemshuku mining district, tsavorite occurs in quartz veinlets and gashes that cut vanadium-bearing graphitic gneiss, and in potato-like nodules. The nodules generally produce crystals with visible fractures. A deposit formed via erosion and redeposition of the crystals is located downslope from the primary occurrences.
It has been suggested that tsavorite forms under pressures in excess of 5 kilobars and temperatures above 750 degrees. However, the tanzanite in the Merelani district has been shown to have formed at lower temperatures and pressures, and presumably much later, from hydrothermal fluids rather than from regional metamorphism.
Topaz has the general formula Al2Si04(F,OH)2. Most topaz is colorless, but it can also be pale yellow, pink, orange, brown, blue, green or gray. The pink and reddish hues result from chromium, manganese and iron substitution for aluminum. The brownish and blue colors are primarily due to a variety of defects called color centers within the stones and are greatly enhanced by heat treatment. Irradiated sky-blue topaz began appearing on the market in the mid-1980s; the color is thought to be due to hydroxyl (OH) substituting for oxygen anions at fluorine sites prior to irradiation.
More than 80 world deposits of wellcrystallized topaz are known. Pegmatites, especially those in Minas Gerais, Brazil, produce the bulk of gem topaz. Such pegmatites are typically shallow and enriched in rare-earth elements.
Topaz also occurs as a primary mineral within rhyolite flows. A fluorineenriched belt of Cenozoic rhyolite units rich in topaz occurs in the western United States and Mexico. The topaz occurs within lithophysae (small gaps in volcanic rocks) and more rarely in fractures or within the rhyolite. Textures indicate that the topaz formed over a range of time from early in the extrusive events to later in the process, and at temperatures ranging from 650 to 850 degrees, with most crystals forming at the lower end of this range. However, colorless topaz from Cerro El Gato in the San Luis Potosí area of Mexico has been reported to crystallize at temperatures above 500 degrees from fluids enriched in elements leached from the lava, whereas ambercolored topaz crystallized below 500 degrees from fluids richer in volatile elements, including arsenic.
Neither greisens nor veins are important sources of gem topaz, although one exception may be Ouro Preto in Minas Gerais, Brazil. Much gem topaz is produced from this region, especially the chromium-rich red “imperial” variety. A number of hypotheses have been proposed for the origin of these deposits, ranging from pegmatitic to a hydrothermal vein. The latter is supported by high concentrations of hydroxyl in the topaz.
Two Kinds of Jade
The term “jade” refers to two rock types: jadeitite rock consisting almost entirely of jadeite (NaAlSi2O6), and nephrite, a variety of tremolite-actinolite mineral [Ca2(MgFe)5Si8O22(OH)2] with extremely tiny crystals—so small that they are vague even when viewed microscopically. Jadeitite is harder than nephrite, but the fracture toughness and surface energy of nephrite is approximately twice that of jadeitite. Jadeitite is less common and more valuable than nephrite and is used more in jewelry than in sculptures.
Pure jadeite is white. Green and blue colors are attributed to iron substituting for aluminum in the jadeite crystal structure. The “imperial” emerald-green color is due to chromium replacing as little as 2 to 3 percent of the aluminum sites. A mauve color is attributed to manganese when iron contents are very low. Nephrite rarely displays an intense emeraldgreen color from chromium substitution.
There are only about 14 documented jadeitite occurrences. The most important jadeitite district is the “Jade Tract” in Kachin State, northern Myanmar, where the jadeitite occurs as intrusions or fragments in serpentine-dominated conglomerations of rock, and in alluvial deposits. Another important source of jadeitite is the middle Motagua Valley in Guatemala, where two belts associated with serpentinite oppose each other across the boundary between the North American and Caribbean plates.
The evidence suggests that all jadeite (and many nephrite) deposits form at the edges when fluids interact with serpentinizing peridotites and surrounding rocks, from a depth of 50 kilometers up to the near surface.
Most nephrite is produced by contact or infiltration metasomatism in two different ways: the replacement of serpentinite by calcium at contacts with more silicic rock, and the replacement of dolomite by silicic fluids associated with granitic magmas. The serpentinereplacement deposits are larger and more abundant than the dolomitereplacement types. The mineral assemblages for nephrite associated with serpentinite suggest that they underwent metamorphism and metasomatism at temperatures from about 300 to 350 degrees down to perhaps 200 to 100 degrees. The most important serpentine-replacement deposits are found at mines in northern British Columbia and in the East Sayan Mountains in Siberia. Dolomite-replacement mines occur in the Kunlan Mountains of China and the Cowell province in southern Australia.
High-Tech Gem Hunting
Many gems are still mined in remote places by individuals, but the gem industry has become modern and systematic in its methods of discovering new deposits. Since the 1980s, when images from the Landsat Earthobserving satellite were declassified and made cheaply available, high-tech prospectors have been able to use its visible-spectrum photos, as well as spectroscopic images of areas without vegetation to directly map minerals. Newer satellites have greatly improved image resolution.
In Canada, satellite and aerial images have been used to search for potential sites of diamond-containing kimberlites, as these volcanic formations have a different chemical and magnetic reading than that of the surrounding rock. Searching for magnetic signatures helps particularly in the discovery of kimberlites that are covered over or underwater. In 2002, 300 million acres of possible kimberlite area in Canada were identified. This region was narrowed down to 8 million acres by direct sampling. Exploration companies have now identified 22 kimberlites that are expected to be high in diamond content.
Other physical and chemical properties of rocks can be used for exploration. Cédric Simonet and his colleagues at Akili Mineral Services in Kenya have found that rocks associated with ruby deposits in that country have a lower electrical resistance and radioactivity when compared to the surrounding host rocks.
Another approach is geochemistry, looking for telltale mineral compositions that are the signature of certain gem deposits. In Colombia, host rocks that are high in sodium but depleted in lithium, potassium, beryllium and molybdenum have been found to be good indicators of emerald deposits. Surveys have tested the streams and sediments that drain from known emerald deposits and found them to be good indicators of the composition of the gem-laden rock, demonstrating that streambeds could be tested to find nearby emerald deposits. Anomalous sodium content in sediments has been successfully used to find several new emerald occurrences.
Berylometers are helpful instruments for emerald and beryl searches. These machines use antimony-sourced gamma radiation to excite a response from beryllium atoms, so its content can be mapped in host rocks. There are two models currently available; one weighs only about five pounds but takes several minutes to read results and has a relatively high detection limit. The other weighs 38 pounds, but is much faster and more sensitive. Their respective drawbacks currently limit the application of these machines to field studies.
The vast expanses of black shale in northwestern Canada would seem to have potential for Colombian-style emerald mineralization. Our group is currently studying how to go about exploring for such deposits, starting with geochemical analyses that look for regions high in sodium and low in beryllium and potassium. We are also looking at structural geology—regions that have tear faults and associated thrusting upward of rock that is linked to décollement planes.
Other studies have shown that for pegmatite sources of emerald, the pegmatites must be fractionated (their constituent elements separated based on their different solubilities) to become enriched enough to reach beryllium saturation. Ratios of potassium to rubidium and rubidium to strontium are some of the geochemical signatures that can be used to find such pegmatites. Satellite imagery may help to identify plutons that have been buried and overlooked.
However, there must be some caution used in applying technology to the search for gems. Our group has found that some commercial analyses of samples that are looking for beryllium and chromium may not adequately dissolve the mineral phases containing these elements, leading to inaccurate readings. In addition, we have found that the use of mass spectrometry to look for beryllium, which is a very light element, in an analytical program that includes numerous heavy elements may decrease the sensitivity of the beryllium analyses.
As technology improves, it may become easier to locate regions where gemstones could be found. Will this make them less rare, and perhaps less valuable? It’s hard to say what the market outcome may be, but an increase in discovery will only make gems more precious to geologists, who value above all the information the crystals can impart about the inner workings of the Earth.