Diamonds - Mineralogy, Identification & Geology

Parcel of rough, gem-quality diamonds recovered from the Kelsey Lake diamond mine,
State Line district, Colorado (photo courtesy of Howard Coopersmith).

Recently, I've received inquiries from prospectors wanting to find out how to identify raw diamonds. One of my favorite stories concerned a rock hound who used his truck's windshield to test diamonds. In this method, all diamonds were thought to scratch his windshield. I suspect after not being able to see out his window, he finally contacted me to find out why he was finding so many diamonds. Unfortunately, windshield glass has a hardness of 5.5 to 6.5 which means that most silicates (including quartz) will scratch glass. This is why people in dusty regions of the country have to replace their car windows so often. So this method is useless.

For the amateur, I recommend using a diamond detector or diamond detective. These simple instruments test the surface conductivity of a mineral and either reads as "diamond" or as "synthetic". Most diamonds will read as diamond and all other minerals should read as synthetic. This will save most prospectors several years of college courses in mineralogy, etc.

INTRODUCTION

Diamonds are extraordinary minerals with extreme hardness and inherent beauty often sought for personal adornment and industrial use. Because the genesis of this unique mineral requires extreme temperature and pressure, natural diamond is rare.  So rare that some diamonds are the most valuable commodity on earth based on weight.

Diamonds are mined on several continents. The value of the raw production has resulted in a multi-billion dollar industry. Natural diamond production averages more than 110 million carats annually valued at more than $7 billion for raw stones. Diamond values dramatically increase after faceting and the value again dramatically increases with dressing in jewelry, such that diamond jewelry typically sells for 10 or more times the value of the raw stone. Industrial diamonds, which are of considerably lower value, includes synthetic industrial diamonds. Synthetic industrial diamond production has an average annual value of around $1 billion per year.

MINERALOGY

Diamond consists simply of carbon.  In nature, native carbon may occur as one of the following polymorphs: diamond, graphite or lonsdaleite (Erlich and Hausel, 2002).  The physical differences between these polymorphs are due to different bonds between the carbon atoms in the crystal structure. In diamond, the coordination of the carbon atoms is tetrahedral with each atom held to four others by strong covalent bonds resulting in a mineral with extreme hardness.

In contrast, graphite has six-member hexagonal carbon rings which resonate between single- and double-shared electron bonds.  These graphite sheets are very strong, but the hexagonal rings are stacked and do not share electrons between adjacent sheets, only a residual electrical charge – thus no chemical bonds occur between the sheets, resulting in graphite being soft, and the sheets easily separated.

The hexagonal modification of diamond, known as lonsdaleite, has a closer-packed arrangement of atoms than diamond or graphite resulting in a rare mineral of extreme hardness (Lonsdale, 1971).  Lonsdaleite was initially synthesized at temperatures greater than 1,000°C under static pressures exceeding 130 kbar (Bundy and Kasper, 1967).  DuPont deNemours and Co. obtained the same transformation by intense shock compression and thermal quenching.  Lonsdaleite has since been identified in meteorites and in rare unconventional host rocks: the most notable being the Popigay Depression in Siberia (Erlich and Hausel, 2002). The extreme hardness of lonsdaleite (about 35% greater than diamond) makes it ideal for industrial grinding, but its rarity makes it unattractive for commercial use.

Diamonds are isometric and have high symmetry and cubic, octahedral, hexoctohedral, dodecahedral, trisoctahedral and related habits. Twinning along the octahedral {111} plane is common and often are flattened parallel to this plane producing a crystal habit that appears as flatten, triangular-shaped diamond known as a macle. 

Cubic diamond - note the greasy luster

Cube. Cubes are a relatively uncommon habit for diamond, and when found are primarily frosted industrial stones. Many have been found in placers in Brazil, and a significant percentage of diamonds in the Snap Lake kimberlites of Canada have cubic habit (Pokhilenko and others, 2003). Crystal faces of a cube often exhibit square-shaped pyramidal depressions rotated 45° diagonally to the edge of the crystal face. The cube may also include scattered trigons mixed with pyramidal and other depressions of hexagonal morphology visible with a microscope. 

Octahedron. The octahedron is an eight-sided crystal that has the appearance of two four-sided pyramids attached at a common base. Each pyramid contains four equilateral triangles known as octahedral faces. In nature, an octahedral face will often have positive or negative trigons: small equilateral triangles visible under a microscope. These are growths or etches on the crystal surface that represent a product of disequilibrium during transport to the earth's surface from the initial stable conditions at depth within the mantle. 

Partial resorption of the octahedron will result in different crystal habits including a rounded dodecahedron (12-sided) with rhombic faces. Further resorption may result in ridges on the rhombic faces yielding a 24-sided crystal known as a trishexahedron. Many diamonds from Argyle, Australia, Murfreesburo, Arkansas, and the Colorado-Wyoming State Line district exhibit resorbed crystal habits. Four-sided tetrahedral diamonds are sometimes encountered that are distorted octahedrons (Bruton 1979; Orlov 1977). 

Diamonds commonly enclose mineral inclusions along cleavage planes. These tiny inclusions provide important data on the origin of diamond and may be used to determine the age of the stone or to identify the unique chemistry associated with the genesis of diamond. 

Bort. Bort is poor grade diamond used as an industrial abrasive. It forms rounded grains with a rough exterior and has a radiating crystal habit. The term is also applied to diamonds of inferior quality as well as to small diamond fragments. 

Carbonado is a black to grayish, opaque, fine-grained aggregate of microscopic diamond, graphite, and amorphous carbon with or without accessory minerals. The material is hard, occurs mainly as irregular porous concretions and dendritic aggregates of minute octahedra, and sometimes forms regular, globular concretions. Carbonado is characterized by large aggregates (averaging 8 to 12 mm in diameter) that commonly weigh as much as 20 carats. Specimens of several hundred carats are not uncommon. The density for carbonado is less than that for diamond, and varies from 3.13 to 3.46 gm/cubic cm. Although carbonado had been found in placers in Brazil and Russia, it was not until the 1990s that it was found in situ. Twenty-six grains of carbonado ranging in size from 0.1 to 1 mm were recovered from a 330-lb sample taken from avachite (a specific type of basalt from the Avacha volcano of eastern Kamchatka) (Erlich and Hausel, 2002).

Octahedral diamond (14.2 ct), Kelsey Lake,
Colorado (photo courtesy of Howard Coopersmith

Physical Properties of Diamond 

Parcel of fancy diamond rough, Argyle, Australia (photo by the author)

Diamond exhibits perfect octahedral cleavage with conchoidal fracture. The mineral is brittle and will easily break with a strike of a hammer. Even so, it is the hardest of all naturally occurring minerals and assigned a hardness of 10 on Mohs scale and nearly 8000 kg/mm squared on the Knoop scale. Corundum, the next hardest naturally occurring mineral has a Mohs hardness of 9. Even so, corundum only has a Knoop scale hardness of 1370 kg/mm2. Because of diamond’s extreme hardness as well as excellent transparency, diamond is extensively used in jewelry and has a variety of industrial uses. Diamond’s hardness varies in different crystallographic directions. This allows for the mineral to be polished with less difficulty in specific directions using diamond powder. For example, it is less difficult to grind the octahedral corners off the diamond, whereas grinding parallel to the octahedral face is nearly impossible. 

With perfect cleavage in four directions parallel to the octahedral faces, an octahedron can be fashioned from an irregular diamond by cleaving (Orlov 1977). The specific gravity of diamond (3.516 to 3.525) is high enough that the gem will concentrate in placers with “black sand”. This density is surprisingly high given the fact that it is composed of such a light element. Compared to graphite, diamond is twice as dense due to the close packing of atoms. 

Color. Diamonds occur in a variety of colors including white to colorless and in shades of yellow, red, pink, orange, green, blue, brown, gray and black (Figure 3). Those that are strongly colored are termed fancies. Colored diamonds have included some spectacular stones. For example, at the 1989 Christie's auction in New York, a 3.14-carat Argyle pink sold for $1.5 million. More recently, a 0.95-carat fancy purplish red Argyle diamond sold for nearly $1 million (US). The world’s largest faceted diamond, a yellow-brown fancy known as the 545.7-carat Golden Jubilee (Harlow, 1998), is considered priceless. Possibly the most famous diamond in the world, the 45-carat Hope, is a blue fancy. 

In most other gemstones, color is the result of transition element impurities; but, this is not the case for diamond. Color in many diamonds is related to nitrogen and boron impurities or is the result of structural defects. Diamonds with dispersed nitrogen may produce yellow (canary) gemstones. If diamond contains boron it may be blue, such as the Hope diamond. The Hope was found in India; although many natural blue diamonds have come from the Premier mine in South Africa. Blue diamond with trace boron are semiconductors. Natural irradiation may result in blue coloration in some diamonds (Harlow, 1998). 

The most common color is brown. Prior to the development of the Argyle mine in Australia in 1986, brown diamonds were considered industrial. But due to Australian marketing strategies, brown diamonds are now highly prized. The lighter brown stones are labeled champagne and darker brown referred to as cognac. Yellow is the second most common color and are referred to as “Cape” diamonds in reference to the Cape Province. When the yellow color is intense, the stone is referred to as “canary”.

Pink, red and purple diamonds are rare. The color in these is due to tiny lamellae (referred to as pink graining) in an otherwise colorless diamond. The color lamellae are thought to be a result of deformation of the diamond structure.

Even though there are many green diamonds, few are faceted, primarily because most have a thin surface covering clear diamond such that if the stone is faceted, the green layer is removed. Faceted green diamonds are so rare that only one is relatively well known (the 41-carat Dresden Green), and is thought to have originated in India or Brazil. The color in most green diamonds is the result of natural irradiation. Other green diamonds may result from hydrogen impurities. Another variety, known as a green transmitter produces strong fluorescence that tends to mask the yellow color of the stone. Other colors include rare orange and violet diamonds (Harlow, 1998). 

One of the better-known black diamonds is the 67.5-carat Orlov. Black diamonds are colored by numerous graphite inclusions, which also make the diamond an electrical conductor. These are difficult to polish due to abundant soft graphite, thus black gem diamonds are uncommon. Opalescent, or fancy milky white diamond, is the result of numerous mineral inclusions and possibly nitrogen defects in the crystal (Harlow, 1998). 

Diamond has high coefficient of dispersion (0.044): the coefficient being the difference in refractive index of two visible light wavelengths at the opposite ends of the spectrum (one blue-violet and the other red). This results in distinct fire in faceted diamond due to high dispersion. Diamond is completely transparent to a broad segment of the electromagnetic spectrum. It is also transparent to radio and microwaves. Colorless diamonds are transparent to visible light wavelengths extending into the ultraviolet, and a few rare diamonds are transparent over much of the ultraviolet spectrum.

Diamond has a luster described as greasy to adamantine that is related to its high refractive index (IR=2.4195) and density. Such high density greatly diminishes the speed of light. For example, the speed of light in a vacuum is 186,000 mi/sec, but in diamond, it is only 77,000 mi/sec (Harlow, 1998). 

Many diamonds are luminescent: approximately one-third of all diamonds luminance blue when placed in ultraviolet light. In most cases, luminescence will stop when the ultraviolet light is turned off (known as fluorescence). Diamonds fluoresce in both long- and short-wave ultraviolet light. The fluorescence is usually greater in long wave and diamond may appear blue, green, yellow or occasionally red. However, fluorescence is generally weak, and it may not be readily apparent to the naked eye. In some cases, light emission is still visible for a brief second after the ultraviolet light source is turned off (known as phosphorescence). Some diamonds may also show brilliant phosphorescence when rubbed or exposed to the electric charge in a vacuum tube; or when exposed to ultraviolet light (Dana and Ford, 1951). 

At room temperature, diamond is four times as thermally conductive as copper, even though it is not electrically conductive. Because of the ability to conduct heat, diamond has a tendency to feel cool to the lips when touched, since the gemstone conducts heat away from the lips. This is why diamonds have been referred to as “ice”. GEM testers (about the size of a pen) are designed to identify the unique thermal conductivity of diamond and distinguish it from other gems and imitations. 

Diamonds are hydrophobic (non-wetable). Even though diamond is 3.5 times heavier than water, it can be induced to float on water. Because it is hydrophobic, diamond will attract grease, thus providing an efficient method for extracting diamond from ore concentrates (i.e., grease table). Oil, grease, and other hydrocarbons have an affinity for materials that do not contain oxygen (such as diamond). 

Diamonds are unaffected by heat except at high temperatures. When heated in oxygen, diamond will burn to carbon dioxide (considered a pollutant by the EPA- for crying out loud, this is nothing more than plant food - the EPA again provides evidence of its lack of credibility). Without oxygen, diamond will transform to graphite at much higher temperatures (1900°C). Diamonds are unaffected by acids.