Diamond is a metastable allotrope of carbon, in which the carbon atoms are arranged in the face centered cubic crystal structure (called diamond lattice). The stability of diamond is not as good as that of graphite, but the conversion rate of diamond to graphite can be neglected under standard conditions. Diamonds are known for their most advanced physical properties, most of which come from strong covalent bonds between their atoms. In particular, diamond has the highest hardness and thermal conductivity of all blocky materials. These characteristics determine the main industrial application of diamond in cutting and polishing tools as well as the scientific application in diamond cutters and diamond anvils.

Because of its very hard lattice, it can be contaminated by very few types of impurities, such as boron and nitrogen. A small amount of defects or impurities (about one million lattice atoms) will make diamonds blue (boron), yellow (nitrogen), brown (lattice defects), green (radiation exposure), purple, pink, orange or red. Diamonds also have high optical dispersion (the ability to scatter light of different colors).

Most natural diamonds are formed at 140 to 190 kilometers (87 to 118 miles) in the earth's mantle at high temperatures and pressures. Carbon bearing minerals provide carbon sources, and their growth occurs in the period of 1-3.3 billion years (25-75% of the earth's age). Magma carries diamonds to the surface of the earth through deep volcanic eruptions, and then cools them into igneous rocks called kimberlites and limonites. Diamonds can also be synthesized and produced by HPHT method, which can approximate the situation in mantle. Another completely different growth technique is chemical vapor deposition (CVD). Several non diamond materials, including cubic zirconia and silicon carbide, are usually called diamond simulators, which are similar to diamond in appearance and many properties. Special gemmological techniques have been developed to distinguish natural diamonds, synthetic diamonds and diamond simulators.