Diamond is the hardest known natural material (second-hardest material to ultrahard fullerite), and is the most expensive of the two best known forms (or allotropes) of carbon, whose hardness and high dispersion of light make it useful for industrial applications and jewelry. (The other equally well known allotrope is graphite.)

Roughly 49% of diamonds originate from central and southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia. They have been treasured as gemstones since their use as religious icons in India at least 2,500 years ago.

They are commonly judged by the "four Cs": carat, clarity, color, and cut. Diamond is the hardest natural material known, scoring 10 on the relative Mohs scale of mineral hardness and having an absolute hardness value of between 90, 167, and 231 gigapascals in various tests.