Diamond Value, Price, and Jewelry Information

Diamond: Africa (portion of A. V. Gumuchian’s “Spectrum Collection,” New York City; ~ ½ to 5). © Joel E. Arem, PhD, FGA. Used with permission.

Please also see our detailed article on choosing a diamond and grading diamonds.

Optics

Isotropic, index very constant; N= 2.417.

Inclusions

Diamond crystals frequently contain crystals of other minerals.

• Olivine may look like bubbles (rounded crystals), present in single crystals or clusters, often on octahedral faces and aligned parallel to octahedral edges. These are pale green or colorless.
• Garnet is present in single crystals or clusters: brown, orange, yellow, pink, violet-red, lilac, and purple colors have been observed. These are usually pyrope garnets and sometimes reach large size. They are seen frequently in South African diamonds.
• Graphite is present as black inclusions.
• Pyrrhotite, pyrite, pentlandite, ilmenite, and rutile (darkcolored ore minerals) may resemble graphite inclusions; these are typical of diamond from Ghana.
• Diamond crystals are often seen as inclusions in other diamonds, usually in perfect crystal forms.
• Chrome diopside is present as emerald-green, well-formed crystals. Also seen in South African diamonds is chrome enstatite.
• Chrome spinel in octahedra, sometimes distorted, usually reddish-brown or black; these are commonly seen in Russian diamonds.
• Ruby has also been observed in an eclogitic diamond.
• Cloudlike inclusions are sometimes in the shape of a Maltese cross, and are diagnostic of diamonds from India.

Occurence

Diamond is a mineral formed at very high temperatures and pressures, deep within the earth. Synthetic diamond is produced at pressures as high as 100,000 atmospheres (equivalent to 200 miles of rock!) and temperatures around 5,000° C. These conditions may approximate those of natural diamond formation.

Diamond formed at depth is apparently “exploded” to the surface in fissures that become circular near the surface and are known as “pipes.” The mineralogy of the rocks in these pipes, known as kimberlite, is unusual and unique and reflects high pressure of formation. Diamond is found in kimberlites and also in alluvial deposits (streams, river channels, beaches, deltas, and former stream beds) derived from kimberlite weathering and erosion.

• South Africa: Diamonds were first discovered on the shores of the Orange River. After several “rushes,” abundant “diamond fever,” and a turbulent period of changing ownership, nearly all the deposits were under control of De Beers Consolidated Mines, Ltd. by 1888. De Beers is now part of Anglo American, a huge conglomerate that also owns the rich gold mines of the Rand in South Africa. South African diamonds are among the world’s most famous, and such mines as Premier, Jagersfontein, Bultfontein, Dutoitspan, and Wesselton are famous for their output. South Africa is still a world leader in diamond production, but large stones are becoming very scarce.

• Other African countries: Diamonds are found in many parts of Africa. Zimbabwe is noted for alluvial deposits. The huge production of very fine stones from Angola has been interrupted by political problems. Ghana produces diamond from gravel beds, mostly industrial; some are gem quality. The Ivory Coast and Republic of Guinea produce alluvial diamonds. A large deposit is known in Namibia where the Orange River enters the Atlantic Ocean. Huge machines work enormous beach deposits in Namaqualand, and other spots along this coast. Central African Republic produces diamonds associated with gravel beds. Alluvial diamonds occur in Zaire and especially in Sierra Leone. The Sierra Leone diamonds are among the world’s finest. They occur in river gravels, are often very large, and are of top gem quality. Occasional stones are found in Tanzania; John Williamson found a large pipe in 1935, and some fine diamond has been recovered from this deposit. Other African sources include Lesotho and Botswana.
• India: The first major historical source of diamonds, and also the source of many of the largest and most famous gems (including the Hope diamond). Mines are in Golconda, Andhra Pradesh (Hyderabad), Kollur, and other localities. Indian diamonds are primarily alluvial, found in sandstones and conglomerates or gravel deposits.
• Brazil: Produces a large quantity of diamond, but little of good gem quality. The Diamantina district was opened in 1725, and diamond also comes from Bahia, Minas Gerais, Matto Grosso, and other states. Diamond in Brazil occurs in a variety of rock types and also alluvial deposits. Most of the stones are small in size but an occasional large, fine gemstone is found. Bahia produces black microcrystalline diamond known as carbonado. The largest of these found weighed 3078 carats.
• Borneo and Indonesia: Small alluvial deposits. Most stones are small (less than I carat). Diamonds from Borneo have been reported to be harder than those from other deposits.
• Venezuela: A substantial alluvial production, mostly of small, yellowish crystals.
• Russia: One of the leading world suppliers of diamonds. The country is rich in pipes (several hundred have been found), some of very large size (such as the famous “Mir” pipe). However, most Russian diamonds are very small, severely limiting the value of the production. A high percentage of crystals is of good color and transparency, and the production is substantial enough to be a major factor in the world diamond market. All the pipes are located in Siberia, where weather conditions make mining both difficult and expensive.
• Australia: As long ago as 1972 it was realized that the geology of northwestern Australia was strikingly similar to that of South Africa’s diamond region. A group called the Ashton Joint Venture Partners started to explore this region and found kimberlite pipes in 1976. A diamondiferous pipe was then found at Ellendale in 1977 and a rich field of alluvial diamonds at Smoke Creek in 1979. An immense pipe known as AK-1, south of Lake Argyle, is being developed; this pipe is complex with an elongated surface outcrop. AK-I, discovered in 1979, contains more than 100 million tons of kimberlite, much of it with an unusually high grade of 7 carats per ton. It was estimated that Smoky Creek plus AK- I could add as much as 50% to the world’s known diamond reserves. However, Argyle diamonds tend to be small and low in quality (5% gem, 40% low-grade gem, 55% industrial). South African diamond will therefore continue to dominate the world gemstone market. However, Australia (in carat production) is expected to become the world’s largest diamond producer.
• United States: The only significant diamond deposit in North America is at Murfreesboro, Arkansas. This is a very large pipe, which has never been systematically developed and might be extremely rich. It is on government owned land and has been worked surficially only by tourists who pay a small fee for the privilege of digging. The largest crystal found here weighed 40.23 carats and was named the “Uncle Sam” diamond.
• Alluvial diamonds have been found throughout the United States, presumably carried south by waters flowing from Canadian glaciers thousands of years ago. The Canadian source pipes have never been discovered, however. Large diamonds found in Virghua include the “Dewey” (1885,23.75 carats) and the “Punch Jones’ (34.46 carats).

Diamond: South Africa: at sorting office of DeBeers Co., Kimberly (rough crystals, ~ 0.25 to 5). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Comments

Diamond is the most romanticized and heavily marketed of all gemstones. Nearly every jewelry establishment handles diamonds, even if it has no other gemstones in stock. The annual world production of diamonds is on the order of 10 tons. Of course, only a small percentage of this is gem quality, but diamond of very fine quality is nowhere near as scarce as equivalently high-quality ruby or emerald.

Name

From the Greek word adamas, meaning the hardest steel, and hence the hardest gemstone.