Diamond Overview

Diamond is a form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At room temperature and pressure, another solid form of carbon known as graphite is the chemically stable form of carbon, but diamond almost never converts to it. Diamond has the highest hardness and thermal conductivity of any natural material, properties that are utilized in major industrial applications such as cutting and polishing tools. They are also the reason that diamond anvil cells can subject materials to pressures found deep in the Earth.

Occurrence and Mining

Diamonds are found in alluvial (loose earthy material deposited by running water) formations and in volcanic pipes, filled for most of their length with blue ground or kimberlite, an igneous rock consisting largely of serpentine. At the surface the blue ground is weathered to a clay called yellow ground. Because the arrangement of atoms in diamond is extremely rigid, few types of impurity can contaminate it (two exceptions being boron and nitrogen). Small numbers of defects or impurities (about one per million of lattice atoms) color diamond blue (boron), yellow (nitrogen), brown (defects), green (radiation exposure), purple, pink, orange, or red. Diamond also has relatively high optical dispersion (ability to disperse light of different colors). Most natural diamonds have ages between 1 billion and 3.5 billion years. Most were formed at depths between 150 and 250 kilometres (93 and 155 mi) in the Earth’s mantle, although a few have come from as deep as 800 kilometres (500 mi). Under high pressure and temperature, carbon-containing fluids dissolved various minerals and replaced them with diamonds. Much more recently (tens to hundreds of million years ago), they were carried to the surface in volcanic eruptions and deposited in igneous rocks known as kimberlites and lamproites. Man-made diamonds can be grown from high-purity carbon under high pressures and temperatures or from hydrocarbon gas by chemical vapor deposition (CVD). Materials such as cubic zirconia and silicon carbide (also known as moissanite) can be used to create imitation diamonds for use in jewelry.

Quality Factors

Subtle differences in color can dramatically af¬fect diamond value. Two diamonds of the same clarity, weight, and cut can differ in value based on color alone. Even the slightest hint of color can make a dramatic difference in value. Diamonds come in many colors. Diamonds that range from colorless to light yellow and brown fall within the normal color range. Within that range, colorless diamonds are the rarest, so they’re the most valuable. They set the standard for grading and pricing other diamonds in the normal color range.

Few things in nature are absolutely perfect. This is as true of diamonds as anything else. Diamonds have internal features, called inclusions, and surface irregularities, called blemishes. Together, they’re called clarity characteristics. Clarity is the relative absence of inclusions and blemishes. Among other things, blemishes include scratches and nicks on a diamond’s surface. Inclusions are generally on the inside, and some might break the surface of the stone. Sometimes, tiny diamond or other mineral crystals are trapped inside a diamond when it forms. Depending on where they’re located, they might remain after the stone has been cut and polished, and they can affect a diamond’s appearance.

A beautifully finished diamond is dazzling, with every facet displaying the craftsman’s skill and care. When a diamond interacts with light, every angle and every facet affects the amount of light returned to the eye. This is what gives it its face-up appearance.A diamond’s proportions determine how light performs when it enters the diamond. If light enters through the crown and goes out through the pavilion, the diamond will look dark and unattractive. Diamonds with different proportions and good polish make better use of the light, and will be bright, colorful, and scintillating.
Many goods are sold by weight—by the kilogram, ounce, pound, or ton. Even people who have never bought a diamond are used to the idea that weight and price are related. They understand that a larger diamond is probably more valuable than a smaller one. But there are two things that often surprise people when they start learning about diamonds and carat weight. The first is the precision with which diamonds are weighed. Diamond weights are stated in metric carats, abbreviated “ct.” One metric carat is two-tenths (0.2) of a gram—just over seven thousandths (0.007) of an ounce. One ounce contains almost 142 carats. A small paper clip weighs about a carat. The metric carat is divided into 100 points. A point is one hundredth of a carat. Diamonds are weighed to a thousandth (0.001) of a carat and then rounded to the nearest hundredth, or point. Fractions of a carat can mean price differences of hundreds—even thousands—of dollars, depending on diamond quality.

Identification

Formula Mass

12.01 g/mol

Color

Typically yellow, brown, or gray to colorless. Less often blue, green, black, translucent white, pink, violet, orange, purple, and red.

Crystal habit

Octahedral

Cleavage

111 (perfect in four directions)

Luster

Adamantine

Streak

Colorless

Specific gravity

3.52±0.01

Density

3.5–3.53 g/cm3

Melting point

Pressure dependent

Dispersion

0.044

Optical properties

Isotropic

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