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Icosahedral diamond crystals (IDCs) were found in chemical vapour deposited diamond by a field emission scanning electron microscope, using. YG13 (cemented tungsten carbide containing 13% of Co) as a substrate. It is found that the nanometer sized IDCs are in the shape of a nearly perfect icosahedron and the micrometer sized IDCs are in the shape of a icosahedron with dimples or grooves. A simulation on atomistic scale is performed by assuming a nucleation - growth model in order to elaborate the mechanism of IDC formation. Results show that the nucleus of IDCs is dodecahedrane, C20H20. For the tetrahedral cubic packing, the dihedral angle of {111} facets of IDCs is 2.87 degrees larger than that of the normal {111} twin plane. The geometric position of the bilateral atoms on the {111} twin plane of IDCs is mismatched, resulting in a distorted twin plane (DTP) in IDCs. The high energy of DTP induces the dimples or grooves that occurred in IDCs., QC 20110428