The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Isometric; Crystals sometimes sharp octahedra, dodecahedra, and combinations with other forms. Crystals modified, often rounded and distinguished by the presence of triangular-shaped pits on octahedral faces (once believed to be due to etching, these 'trigons'' are currently believed to be a result of the growth process).|
|Colors||Colorless, gray, shades of yellow, brown, pink. green, orange, lavender, blue. black; rarely red|
|Hardness||10. Diamond is the hardest natural substance and easily scratches any other mineral. The difference in hardness between diamond and corundum (9) is very much greater than that between any other two minerals on the Mohs scale.|
|Density||3.515; Carbonado 2.9-3.5|
|Cleavage||Perfect 1 direction (octahedral). Brittle. In spite of its great hardness, diamond can be split easily along octahedral planes. This feature is useful in cutting, since cleaving a large diamond saves weeks of laborious sawing. The cleavage also makes it possible for diamonds to be chipped in wear.|
|Dispersion||0.044. This high dispersion in a colorless diamond creates the "fire" that is the source of the diamond's attractiveness.|
|Stone Sizes||The largest rough diamonds ever found include the Cullinan (3106 carats, white, South Africa, 1905); the Excelsior (995.2 carats. white, South Africa, 1893), the Star of Sierra Leone (968.8 carats, white, Sierra Leone, 1972), and the Great Mogul (787.5 carats, white, India, 1650). A fine yellowish octahedron of 616 carats is on display at the Mine Museum in Kimberley, South Africa, found in 1975. The world's largest uncut diamond, an 890 carat "fancy intense golden yellow" is owned by the Zale Corp. The stone is African in origin, but from an undisclosed source. If cut, it could yield a finished stone of 600 carats, which would then become the world's largest polished diamond. The Zale diamond is the fourth largest rough ever found. The largest cut stones include: Cullinan I (530.2, white, pear shape, in the British Crown Jewels), Cullinan II (317.4, white, cushion, British Crown Jewels), Great Mogul (280.0, white, dome-shape, location unknown), Nizam (277.0, white, table-cut, was in India in 1934), Jubilee (245.35, white, cushion, privately owned, Paris), and the Orloff 189.6, white, rose-cut, Russian Diamond Fund in the Kremlin).|
|Luminescence||Many diamonds fluoresce blue to violet, with fluorescence sometimes in zones (often a result of twinning). The effect is sometimes strong enough to be visible in daylight. Yellow stones sometimes fluoresce yellow-green. Some pink diamonds from India fluoresce and phosphoresce orange. The famous Hope diamond, deep blue in color, phosphoresces deep red. Most fluorescence occurs in LW; the SW reaction is weaker and the same as LW. Many diamonds fluoresce bluish white in SW. Blue-fluorescing diamonds may phosphoresce yellow (an "afterglow" reaction). The various diamonds have been organized into types, with varying UV transparency. Type 1: Transparent to all wavelengths down to about 3000 Å; this type contains nitrogen and is further subdivided into Types Ia and Ib. Type la represents the majority of all diamond, and the nitrogen is in the form of platelets. About 0.1% of Type I diamonds are Type lb, in which the nitrogen is dispersed throughout the crystal. Type II: Transparent all the way to 2250 Å; this type contains aluminum. Type IIa does not phosphoresce in SW and contains little nitrogen. Type lIb has bluish phosphorescence in SW and is also electrically conductive. Nitrogen in these diamonds is absent or very scarce.|
|Spectral||The absorption spectra of various colored diamonds are quite distinctive and useful, especially in distinguishing irradiation-colored diamond from natural colored stones. The colored diamonds can be grouped into several series: Cape Series: Colorless to yellow diamonds that fluoresce blue. Strong lines at 4155, 4785, 4650, 4520, 4350, and 4230. Most lines are hard to see. Brown Series: Brown, green, and greenish yellow diamonds that fluoresce green. Strong line at 5040 plus weak lines at 5320 and 4980. Yellow Series: Colorless, brownish yellow or yellow and yellow-fluorescing. This series includes the true "canary" yellow diamonds. No discrete spectrum but sometimes a weak line at 4155. Type Il-B Blue: No absorption spectrum. Pink diamonds show the so-called "cape'' absorption line at 4150 and a broad, diffuse band centered at 5500. The strength of this band correlates with the intensity of color of the diamond.|
|Enhancements||Some colors produced by irridation, common. Laser drilled to remove inclusions, common. Cracks filled with glass, occassional.|
|Special Care Instructions||None|
|Formula||C (carbon ). Essentially pure with only minor traces of impurities.|
Please also see our detailed article on choosing a diamond and grading diamonds.
Isotropic, index very constant; N= 2.417.
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.
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 5000°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.
- USSR: Russia is 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 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.
From the Greek word adamas, meaning the hardest steel, and hence the hardest gemstone.