Diamond is a colourless exceptionally hard mineral (but often tinted yellow, orange, blue, brown, or black by impurities), found in certain igneous rocks esp. the kimberlites of South Africa. It is used as a gemstone, as an abrasive, and on the working edges of cutting tools. Composition: carbon. Formula: C. Crystal structure: cubic. Name derived from the Greek adamas meaning invincible. Relative density 3.50- 3.52. Cleavage eminent along octahedral faces. Fracture conchoidal. Tenacity brittle. Lustre brilliantly adamantine. Refractive index 2.417- 2.419. Birefringence and pleochroism: none (cubic). Dispersion strong 0.044. Diamond crystals form as cubes, octahedra , the most common gem form, and dodecahedra. Colourless to yellow diamonds which have a strong absorbtion line in the violet end of the spectrum are members of the Cape series.
