In mineralogy, diamond is the allotrope of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. Its hardness and high dispersion of light make it useful for industrial applications and jewelry. It is the hardest known naturally-occurring mineral. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds (known as Type-II diamonds) that are harder than the diamonds used in hardness gauges. Presently, only aggregated diamond nanorods, a material created using ultrahard fullerite (C60) is confirmed to be harder, although other substances such as cubic boron nitride, rhenium diboride and ultrahard fullerite itself are comparable.
Diamonds are specifically renowned as a material with superlative physical qualities; they make excellent abrasives because they can be scratched only by other diamonds, borazon, ultrahard fullerite, rhenium diboride, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain their lustre. Approximately 130 million carats (26,000 kg (57,000 lb)) are mined annually, with a total value of nearly USD $9 billion, and about 100,000 kg (220,000 lb) are synthesized annually.
The name diamond derives from the ancient Greek ἀδάμας (adamas) "invincible", "untamed", from ἀ- (a-), "un-" + δαμάω (damáō), "to overpower, to tame". They have been treasured as gemstones since their use as religious icons in ancient India and usage in engraving tools also dates to early human history. 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. They are commonly judged by the “four Cs”: carat, clarity, color, and cut.