The lustre of well crystallised Diamonds is adamantine in a splendant degree. From this high lustre all grades may be found to greasy and dull. Water-worn stones often show almost a metallic lustre. Crystals are usually transparent, while Bort is only translucent, and Carbonado is opaque. The degree of transparency varies greatly with the condition of the surface. Thus water-worn crystals may be likened to ground glass—seemingly translucent only; but if their surfaces be polished, as in the process of cutting, the stone may be found to be transparent in a high degree. Stones which show more than a certain degree of cloudiness cannot be utilised as gems.
The refractive index is high, and dispersion is very high. Des Cloizeaux gives the following value of n—
Red. Yellow. Green.
2-4135 2-4195 2-4278
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