Lonsdaleite, also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice, as opposed to the cubical lattice of conventional diamond. It is found in nature in meteorite debris; when meteors containing graphite strike the Earth, the immense 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 ordinary diamond.

Dame Kathleen Lonsdale was an Irish crystallographer, pacifist, and prison reform activist. She proved, in 1929, that the benzene ring is flat by using X-ray diffraction methods to elucidate the structure of hexamethylbenzene. She was the first to use Fourier spectral methods while solving the structure of hexachlorobenzene in 1931. During her career she attained several firsts for female scientists, including being one of the first two women elected a Fellow of the Royal Society (FRS) in 1945, first female professor at University College London, first woman president of the International Union of Crystallography, and first woman president of the British Association for the Advancement of Science.