Precious gemstones

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 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. 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.

Colour 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.

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 needle-like inclusions of a titanium-oxide mineral called rutile, or other iron or iron-titanium oxide phases, aligned along crystallographic planes and parallel to the hexagonal faces at 60 degrees.

American Scientist