Abstract

Naturally occurring diamond is a relatively rare polymorphic form of carbon characterized by a three-dimensional arrangement of tetrahedrally coordinated carbon atoms. Both natural and synthesized diamond have the highest hardness of all known materials, the highest thermal conductivity at room temperature, a high refractive index and optical dispersion, a low thermal expansion, and a relatively high inertness to chemical attack. This unique combination of properties permits diamond to be foremost in certain applications: as a highly prized gemstone; industrially as an important abrasive material unsurpassed in certain cutting, drilling, sawing, machining, grinding, and polishing operations for many materials; and in electronic and optical applications as a heat sink and window material, respectively. Upstream exploration has sometimes led to the discovery of kimberlite or lamproite pipes. These structures are the principal source of natural diamonds, and there are > 6000 occurrences of them in the world. Only a small number contain a high enough concentration of diamonds to warrant mining. Alluvial diamonds are recovered from streambeds by panning or washing techniques or, more productively, by removal from pockets in the streambed where the water has concentrated them. High quality gemstones are recovered from ocean deposits off the western coast of southern Africa. A modern method for the separation of diamond crystals from rock uses the emission of light by luminescence of diamond when exposed to X-rays. The diamonds are then separated mostly by hand into two main categories, gem and industrial. Besides the single crystals usually implied in a discussion of diamond, the latter also occurs in nature in the form of polycrystalline aggregates. These can be classified into two types based on origin: carbonado and ballas. Diamond is nominally pure carbon with a ¹²C/¹³C ratio of ~99:1. Although other elements are often reported in analyses, only boron and nitrogen are considered to be truly substitutional in the lattice. The classification of diamonds into four types (Ia, Ib, IIa, IIb) is based primarily on the presence or absence of nitrogen and boron and their location in the lattice. Diamond is the hardest material known because of its combination of a three-dimensional arrangement of tetrahedrally coordinated CC bonds with a bond distance of 0.154 nm. The Knoop hardness (K) is in the range of 68.7–98.1 kN/m² (7000–10,000 kgf/mm²). Although hard, diamond is also very brittle. The high refractive index (2.42 at 589.3 nm) and dispersion (0.044) are the basis for the “brilliance” and “fire” of a properly cut gemstone. The optical transmission out to 10.6 µm for type IIa diamonds makes possible windows for CO₂ lasers and for devices such as were used in the Venus probe.