False-colour three-dimensional plot of the crystal structure of lonsdaleite, a form of diamond with stacking faults. Diamond is a form of carbon, and is normally found as a perfect triclinic crystal. The regularity of the crystal means that diamond is the hardest known mineral. In this view along one of the cleavage planes, the irregularity of the packing is evident, a perfect diamond structure would consist of regular hexagonal patterns. Lonsdaleite was named after Dame Kathleen Lonsdale (1903-71), British crystallographer and the first woman elected a Fellow of the Royal Society in London.