Diamond is a metastable allotrope of
carbon, where the carbon atoms are arranged in a variation of the
face-centered cubic crystal structure called a diamond lattice. Diamond
is less stable than graphite, but the conversion rate from diamond to
graphite is negligible at standard conditions. Diamond is renowned as a
material with superlative physical qualities, most of which originate
from the strong covalent bonding between its atoms. In particular,
diamond has the highest hardness and thermal conductivity of any bulk
material. Those properties determine the major industrial application of
diamond in cutting and polishing tools and the scientific applications
in diamond knives and diamond anvil cells.

Because of its
extremely rigid lattice, it can be contaminated by very few types of
impurities, such as boron and nitrogen. Small amounts of defects or
impurities (about one per million of lattice atoms) color diamond blue
(boron), yellow (nitrogen), brown (lattice 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 are formed at high temperature and pressure at depths
of 140 to 190 kilometers (87 to 118 mi) in the Earth’s mantle.
Carbon-containing minerals provide the carbon source, and the growth
occurs over periods from 1 billion to 3.3 billion years (25% to 75% of
the age of the Earth). Diamonds are brought close to the Earth’s surface
through deep volcanic eruptions by magma, which cools into igneous
rocks known as kimberlites and lamproites. Diamonds can also be produced
synthetically in a HPHT method which approximately simulates the
conditions in the Earth’s mantle. An alternative, and completely
different growth technique is chemical vapor deposition (CVD). Several
non-diamond materials, which include cubic zirconia and silicon carbide
and are often called diamond simulants, resemble diamond in appearance
and many properties. Special gemological techniques have been developed
to distinguish natural diamonds, synthetic diamonds, and diamond
simulants. The word is from the ancient Greek adámas
“unbreakable”.