Page:Popular Science Monthly Volume 8.djvu/219

Rh prismatic spectrum. All limpid and white gems must be cut according to this rule, but with colored stones the case is different, for here perfection of color is to be attained, and brilliancy is a secondary consideration. Hence, a fine ruby or sapphire may be decidedly thin, and yet be a gem of great beauty and value.

The process of rifting diamonds by splitting them in their cleavage-planes was known long ago to the Hindoos, but was forgotten to modern lapidaries till revived by Wollaston not many years ago. By this means masses of the crystal may be removed to escape a flaw or remove a spot. Some diamonds of the spheroidal form are deficient

in cleavage-planes, and are quite impracticable for cutting; others have a concentric arrangement of the planes of cleavage, as though crystallization radiated from the centre, and it is very difficult to polish them. The Hindoos avail themselves of the natural cleavage