Lonsdaleite

Lonsdaleite (named in honour of Kathleen Lonsdale), also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond.

Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967) by compressing and heating graphite either in a static press or using explosives. It has also been produced by chemical vapor deposition, and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at temperature 110 °C (230 °F).

It is translucent, brownish-yellow in color, and has an index of refraction of 2.40 to 2.41, a specific gravity of 3.2 to 3.3, and a Mohs hardness of 7–8. The Mohs hardness of diamond is 10, and the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material. A simulated pure sample has been found to be 58% harder than diamond.