There are a number of forms adopted by the lapidaries for these gems, but the two principal ones are the brilliant and the rose. The first pattern, which was invented in Europe in the seventeenth century, is perhaps the best of all to call forth the powers of the gem. The second is of unknown age, and was practised by the Hindoos in ancient times. It affords the largest beams of light for the weight, but it lacks in colored reflections when compared with the brilliant. For the perfection of the rainbow play of hues, it is essential that the facets of the superior and inferior parts of the stone should correspond in exact proportions and at fixed distances, so as to multiply the reflections and refractions, and produce the colors of the prismatic spectrum. Therefore the perfect cutting of the diamond is a mathematical problem, governed by rigid laws. All limpid and white gems must be cut according to this rule, but in case of colored stones the case is far different; for perfection of color is to be attained, and brilliancy is a secondary thought. Therefore a fine ruby or sapphire may be decidedly thin in form, and yet be a gem of great beauty and value. It is very rare we obtain the full perfection of brilliancy and color in the colored gems; for generally the depth of the finest hues prevents a display of the refractive powers of the stone. The process of rifting diamonds by splitting them in their cleavage planes was known long ago to the Hindoos; but, strange to say, it was forgotten by modern lapidaries until 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, and yet preserve material that may be utilized, and which would have been lost by the process of grinding away the exterior. Some diamonds of the spheroidal form are deficient in cleavage planes, and are quite impracticable for cutting; and the fact of the impossibility of polishing them is sufficient proof of their peculiar origin, and that their form is not due to aqueous action. Others, known as bort, seem to be of concentric arrangement, as though crystallization radiated from the centre; and it is very difficult to polish them. The Hindoos avail themselves of this natural cleavage, and form table diamonds by adroitly striking along one of the planes of the stone with a sharp-edged tool, thereby separating the layers of the gem as the slate is rifted by the miner. The operation, which seems so simple, really requires considerable skill and much of that acquired instinct or tact which is best exhibited by our Western Indians, who chip, with marvellous rapidity and certainty, a glass bottle into symmetrical arrow-heads. This method of reducing diamonds by cleavage was early known to the Hindoos, and is mentioned by Tavernier in an unmistakable manner in his published travels.