Diamond

The slightly misshapen octahedral shape of this 
rough diamond crystal in matrix is typical of the mineral
Its lustrous faces also 
indicate that this crystal is from a primary deposit.

In mineralogy, diamond   is an 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 ambient 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.
Diamond has remarkable optical characteristics. Because of its

extremely rigid lattice, it can be contaminated by very few types of

impurities, such as boron and nitrogen. Combined with wide

transparency, this results in the clear, colorless appearance of

most natural diamonds. 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),

which results in its characteristic luster. Excellent optical and

mechanical properties, combined with efficient marketing, make

diamond the most popular gemstone.
Most natural diamonds are formed at high-pressure high-temperature

conditions existing at depths of 140 to 190 kilometers (87 to 120

mi) in the Earth 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 surface through deep volcanic eruptions

by a magma, which cools into igneous rocks known as kimberlites and

lamproites. Diamonds can also be produced synthetically in a

high-pressure high-temperature process which approximately simulates

the conditions in the Earth 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 and synthetic

diamonds and diamond simulants.

History
The name diamond is derived from the ancient Greek αδάμας (adámas), "proper", "unalterable", "unbreakable", "untamed", from ἀ- (a-), "un-" + δαμάω (damáō), "I overpower", "I tame". Diamonds are thought to have been first recognized and mined in India, where significant alluvial deposits of the stone could be found many centuries ago along the rivers Penner, Krishna and Godavari. Diamonds have been known in India for at least 3,000 years but most likely 6,000 years.
Diamonds have been treasured as gemstones since their use as religious icons in ancient India. Their usage in engraving tools also dates to early human history. The popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns.
In 1772, Antoine Lavoisier used a lens to concentrate the rays of the sun on a diamond in an atmosphere of oxygen, and showed that the only product of the combustion was carbon dioxide, proving that diamond is composed of carbon. Later in 1797, Smithson Tennant repeated and expanded that experiment. By demonstrating that burning diamond and graphite releases the same amount of gas he established the chemical equivalence of these substances.
The most familiar use of diamonds today is as gemstones used for adornment, a use which dates back into antiquity. The dispersion of white light into spectral colors is the primary gemological characteristic of gem diamonds. In the 20th century, experts in gemology have developed methods of grading diamonds and other gemstones based on the characteristics most important to their value as a gem. Four characteristics, known informally as the four Cs, are now commonly used as the basic descriptors of diamonds: these are carat, cut, color, and clarity. A large, flawless diamond is known as a paragon.
Natural history
The formation of natural diamond requires very specific conditions—exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars (4.5 and 6 GPa), but at a comparatively low temperature range between approximately 900–1300 °C. These conditions are met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.