III.

Properties

Diamond is the hardest natural substance known. This hardness is exhibited in diamond’s resistance to scratching and its ability to scratch other materials. Steel and glass, for instance, can be scratched by diamond. The Mohs hardness scale, devised by the German mineralogist Friedrich Mohs to indicate relative hardness of substances on a rating scale from 1 to 10, assigns diamond a value of 10. Diamond’s hardness is not a constant quantity but varies even within a single diamond.

Diamonds are crystals composed of carbon atoms. Atoms in a crystal are arrayed in a regular repeating pattern. A crystal’s outward form, bounded by smooth plane surfaces that meet at predictable angles, reflects this internal order. Crystals tend to cleave, or split, along lines called cleavage planes between layers of atoms. In the case of diamond crystals, each carbon atom is bonded to four surrounding carbon atoms. This microscopic arrangement determines the visible shape of diamond crystals, which are generally octahedrons (solid shapes with eight faces). Individual diamond crystals therefore cleave cleanly along planes parallel to the faces of an octahedron.

Two important properties, brilliance and fire, contribute to diamond’s beauty. Brilliance is the fraction of the light that falls on a diamond that the diamond returns to the eyes of an observer—the more light returned, the higher the brilliance. Diamond’s brilliance arises from its index of refraction, which determines the angle at which light is bent as it crosses the boundary between the air and the stone. Fire is the ability of a substance to split white light into rainbow colors—the greater the separation between colors, the greater the fire. Diamond’s fire originates with its dispersion, which is the difference in diamond’s index of refraction for light of different colors. Diamond has both a higher index of refraction and a higher dispersion value than any other natural, transparent, colorless material.

Diamonds exhibit a wide range of transparency and color. Transparency is a measure of the amount of light that passes through a diamond rather than being absorbed. Colorless diamonds, known as white diamonds, are most familiar, but green, blue, red, orange, yellow, and brown diamonds also are known. Structural imperfections or dislocations and the presence of trace elements, mainly nitrogen, cause color in diamonds. Some diamonds luminesce (emit light) when exposed to sunlight or other ultraviolet-light sources. The light the diamonds emit is usually light blue, but yellow, orange, and red luminescence occurs in some stones.

Most diamonds used as gems are single crystals large enough to be easily visible to the eye. Diamond also occurs, however, in polycrystalline forms commonly known as ballas, bort, and carbonado. Ballas is a compact, spherical mass of tiny diamond crystals of great hardness and toughness. Bort is an extremely hard, dark, imperfectly crystallized diamond. The term bort sometimes is also applied to minute fragments of gem diamonds. Carbonado is an opaque grayish or black form of diamond that consists of microscopic crystals and has no cleavage. Ballas, bort, and carbonado are all used industrially, in lapidary (gem-cutting) work, and as a tough coating for the tips of drills and the edges of cutting tools.

Other characteristics of diamonds are frequently useful in identifying the stones and in differentiating between true diamonds and imitations. Because diamonds are excellent conductors of heat, they are cold to the touch and are sometimes called “ice.” Most diamonds do not conduct electricity well, but diamonds do become charged with positive static electricity when rubbed. Diamond resists attack by acids or bases. Since diamonds are a form of carbon, like coal, they will burn, but only when heated to extremely high temperatures.

The density of diamond ranges between 3.15 and 3.53 g/cm³, but the density of pure diamond is always very close to 3.52 g/cm³. Diamond is much denser than crystals composed of elements of similar weight to carbon atoms because the carbon atoms in diamond are packed tightly together. Quartz, for example, is composed of atoms of silicon and oxygen, both of which are heavier than carbon atoms. The density of quartz, however, is only 2.65 g/cm³.