Diamond, crystalline carbon, is polymorphous with graphite, chaoite and lonsdaleite. The hardness is 10 and the SG 3.515. The RI is 2.42 and the dispersion 0.044. Crystals are commonly octahedral, also cubic, dodecahedral or tetrahedral. The unique hardness arises from the three-dimensional covalent linkage of each carbon atom to the four neighbouring ones. The point group is 4/m –32/m, possibly bar –43 m (for an explanation of point groups and related topics see International Tables for Crystallography: Brief Teaching Edition of Space Group Symmetry, second, revised edition, 1988, published at Dordrecht for the International Union of Crystallography by D Reidel Publishing Company, ISBN 9027725780). 

A more comprehensive survey for the International Union is Lima de Faria et al., 1990 (ISBN 079230649X). Here the history of the terms used in crystal descriptions is given and there is a unique and excellent bibliography.

Diamond crystals present special problems in fashioning. Diamond crystals unless very small are not available to collectors; some are well illustrated from the standpoint of the polisher in two books written by Nizam Peters and published by the American Institute for Diamond Cutting, Rough Diamonds: internal and external features, 1998 (ISBN 0966585402). There are many studies of diamond cutting; one fairly recent one is Basil Watermeyer’s The Art of Diamond Cutting, 1994 (ISBN 0412984113). Diamond crystals have an adamantine to greasy lustre and sometimes show curved and striated faces; some crystals are spherical with an internal radial structure. 

Contact twinning with {111} as the twin plane is common and crystals are often flattened on {111}. For explanations of these symbols, which are given in this text only for diamond, see any text of mineralogy, for example Dana’s New Mineralogy (1997) pages xviii–xxxii. The ISBN of this essential text for anyone needing to know about validated mineral species is 0471193100. Crystal faces are commonly marked with trigons whose outline does not echo the edges of the face but is reversed. Etch pits do follow the crystal edges.

Penetration twins, sometimes repeated, are also found. There is a perfect cleavage on {111} and a conchoidal fracture. Diamond shows fluorescence and phosphorescence and is triboelectric. It possesses the highest thermal conductivity of any known substance. Diamond is transparent to translucent and may be colourless, pale to deep yellow, brown, white and blue-white; less common colours reported are orange, pink, green, blue, red and black. 

Those diamonds with strong colours (not merely off-white rather than canary yellow) are known as fancies. Cell data: space group Fd3m. a 3.5595 Z 8. The X-ray powder patterns (given in this text only for diamond) are 2.06 (100), 1.261 (25), 1.0754 (16), 0.8182 (16), 0.8916 (8). Diamond has an SG of 3.52 and varies little; the single RI is 2.42 and the dispersion (fire) 0.044. The ideal polished diamond is cut as a colourless round brilliant – if the ideal combination of brilliance (from the unique hardness) and dispersion is to be achieved. The flashes of colour from the small upper facets surrounding the table are irresistible and are best seen by distant single white spotlights – as in many cathedrals and museums.

Direct sunlight is less effective (less subtle) in producing the best dispersion. Whatever the desired effect from a diamond, the polisher has to pay attention to the facet angles – in a round brilliant, light must not leak from the base or the effects of dispersion will be lost. The flat (spread) brilliants with their large tables may impress (and give the impression of greater weight) but in comparison with the deeper old-cut diamonds with their smaller tables and steeper facet angles their dispersion is low. Old-cut stones show their fire however small they are.