Lonsdaleite is a hexagonal polymorph of the carbon allotrope diamond, believed to form when meteoric graphite falls to Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Lonsdaleite is currently found only in Barringer Crater (also known as Meteor Crater) in Arizona and at Canyon Diablo. It was first discovered in 1967.

Lonsdaleite is also known as "hexagonal diamond." It has an index of refraction from 2.40 to 2.41, a density from 3.2 to 3.3, and a Moh's hardness of 7–8. The Mohs hardness of diamond is 10; the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material.

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