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The rarity of these glittering jewels makes them valuable to geologists studying conditions deep in the Earth

Lee A. Groat

2012-03GroatF1.jpgClick to Enlarge ImageThe 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 colored-gem 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.)

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