A GUIDE TO FINDING GEMSTONES & GOLD

MINERALS

Minerals are solid chemicals with unique physical properties. These properties assist the geologist in mineral identification and classification of groups of minerals into rock types. Some physical properties include specific gravity (or heft), cleavage, crytal habit (or form), mineral associations, rock types and more. In many cases, a mineral may not quite reveal itself to the geologist and may require the use of laboratory equipment such as x-ray diffraction or precession.

Normally, it would be laughable, particularly if it wasn't true. But rumors are surfacing that Harvard University is eliminating its mineralogy and petrology (rock identification) classes in their geology program. I suspect they will have to change the title of this degreed program to 'Almost Geology'. Here are some important minerals that Harvard graduates will no longer be able to identify.

Diamond

Diamonds are isometric (equal-dimensional in their simplest form). They have high symmetry & form cubic, octahedral, hexoctohedral, dodecahedral, trisoctahedral & related crystal habits. Twinning along the octahedral plane is common & often results in flattened crystals parallel to this plane that result in a crystal habit that appears as flatten, triangular-shaped diamond known as a macle.

Cube. Diamond cubes are uncommon. But when found form primarily frosted industrial stones. Many are recovered from placers in Brazil, and a significant percentage of diamonds in the Snap Lake kimberlites of Canada are cubic. 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 that are visible with a microscope.

Octahedron. The octahedron is an 8-sided crystal that has the appearance of two four-sided pyramids attached at a common base. Each pyramid has 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 produced during transport to the earth's surface from the initial stable conditions at depth within the Earth's mantle.

This extraordinary 14.2-carat octahedron was recovered from the Kelsey Lake diamond mine in the State Line district, Colorado. Note the characteristic 8-sided shape (photo courtesy of Howard Coopersmith).

Partial resorption of the octahedron will result in different crystal habits including 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.

Diamonds commonly enclose mineral inclusions along cleavage planes. These tiny inclusions provide data on the origin of diamond and may be used to determine the age of the stone or 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. Carbonado is a black to grayish, opaque, fine-grained aggregate of microscopic diamond, graphite & amorphous carbon. The material is hard, occurs mainly as irregular porous concretions and dendritic aggregates of minute octahedra & sometimes forms regular, globular concretions. Carbonado is characterized by large aggregates (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/cm³. Although carbonado had been found in placers in Brazil and Russia, it was not until the 1990s that it was found in situ.

Physical Properties

Diamond exhibits perfect cleavage with conchoidal fracture. It is brittle and will easily break with a light strike of a hammer. Even so, it is the hardest of all naturally occurring minerals and is assigned a hardness of 10 on Mohs scale & nearly 8000 kg/mm² on the Knoop scale. Corundum, the next hardest naturally occurring mineral has a Mohs hardness of 9. Even so, corundum does not even compare to diamond & 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 & 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 with diamond powder. It is less difficult to grind the octahedral corners off a 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. 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.

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. 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. The world’s largest faceted diamond, a yellow-brown fancy known as the 545.7-carat Golden Jubilee is considered priceless. Possibly the most famous diamond in the world, the 45-carat Hope, is a blue fancy.

Colored diamonds from Australia with cognac diamonds to the right.

In most gemstones, color is the result of minor transition element impurities; however, 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 the diamond contains some boron it may be blue such as the Hope diamond. The Hope was found in India. Most other blue diamonds have come from the Premier mine in South Africa.

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

Resorped textures in diamond (left) and large 620 carat irregular diamond.

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

Even though there are many green diamonds, few are faceted, primarily because most have a thin green surface covering clear diamond such that if the stone is faceted, the green layer is removed. Black diamonds are colored by numerous graphite inclusions, which also make the diamond an electrical conductor. Opalescent, or fancy milky white diamond, is the result of numerous mineral inclusions and possibly nitrogen defects in the crystal.

Some characteristic crystal habits for diamond include (Clockwise), cubic diamond, octahedral diamond, hexagons on diamond surface, trigons on diamond surface, and group of octahedrons.

Diamond has high coefficient of dispersion (0.044) resulting in the distinct fire seen in faceted diamond. Diamonds are unaffected by acids. Diamond has a luster described as greasy to adamantine that is related to its high refractive index (IR=2.4195) and density.

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 CO₂. Without oxygen, diamond will transform to graphite.

CORUNDUM

(Al₂O₃)

Corundum forms hard, hexagonal (6-sided) prisms terminated by pinacoids (flat surfaces) with cleavage and twinning. Of naturally occurring materials, only diamond is harder than corundum (H = 9). It has a relatively high specific gravity (SG = 4 to 4.1) and if were not for cleavage, it would be found more often in placer deposits. Corundum comes in a variety of colors inlcuding gray, white, blue, violet, pink, brown & red. It has adamantine to vitreous luster & when transparent to translucent produces attractive gemstones of sapphire & ruby. Even so, some opaque corundum will exhibit extraordinary color & can be shaped into cabochon gemstones. Corundun may also produce barrel-shaped hexagonal prisms that display striations due to repeated twinning.

Hexagonal corundum in schist (photo courtesy of Art Snoke).

Corundum is an aluminum oxide. Thus it is found in silica-poor aluminum-rich metamorphic rocks known as metapelite, in some aluminum-rich serpentinites and in aluminum-rich skarns (Hausel, Gemstones of the World - Their Geology, Exploration & Discovery, in preparation). Such metapelites may contain a variety of alumino-silicate porphyroblasts such as mica, kyanite, sillimanite, andalusite, vermiculite & cordierite. Corundum is typically found where such schists have been altered by removal of silica which enriches the rock in alumina. In particular, vermiculite schist & aluminum-rich serpentinites are favorable hosts (Kievlenko, 2003; Hausel, 2009).

Over the period of several years, I was able to find nearly a dozen ruby & sapphire deposits simply by searching for vermiculite. At least 30% of vermiculites found in Wyoming contain this gem. Other ruby & sapphire deposits await discovery as I was on the trail of several more ruby deposits in Wyoming before I left the Survey. During a search for kimberlite, I found several tiny rubies & sapphires in stream sediments in the central Laramie Range - this means there are several undiscovered deposits upstream from those samples. In addition, a search began for a significant ruby deposit in the southern Wind River Mountains where corundum as large as 90 carats had been found by prospectors & some mica schist float filled with gem-quality rubies had been picked up. This search terminated when I left the Survey.

The cut mineral above shows distinct parallel lines of parting characteristic of corundum, and the faceted mineral to the left shows very distinct lines (cleavage).

It is very common to mistaken corundum for garnet and vise versa. Garnet does not have cleavage & parting, thus if you see such lines in your 'garnets', you might have rubies instead. Also if you examine enough crystals in a rock and several of the red stones have a 6-sixed crystal habit, it is likely corundum.