But of all the fine red sapphires which are known and proved, there are but few above five karats. The largest one of which we have any definite knowledge is the beautiful gem set in the Toison d’Or of the French Regalia, and which weighs 8³⁄₁₆ karats (= 26 grains troy). The inventory of the French gems in 1791 justly illustrates the comparative rarity and diminutive size of the stone; for in this splendid collection, which had accumulated during a long period of time, and was then the richest in Europe, there were but four red sapphires above five karats, and only five above four karats. This variety is singularly liable to imperfections, and far more so than either the blue or the yellow. It is rare to find a ruby of the pure and characteristic pigeons’-blood tint that does not in some degree exhibit silky and opalescent fibres. This defect, which generally appears as a milkiness in the interior of the gem, is due to minute crystals dispersed throughout the stone, and which become apparent when the mineral is viewed parallel to the primitive axis of the crystal. Hence, in cutting the rough stone, considerable care must be exercised by the lapidary, so as to shape the gem and render its opalescence invisible. Rubies of exquisite color are often rendered comparatively valueless on account of fibres, clouds, and chalcedony-like bands. All the red sapphires, however, are not affected in this way. The blood-red are much more liable than those which have a tinge of blue. We have examined a number of red sapphires perceptibly tinted with violet, which were completely free from internal defects. This opalescence is never possessed by the spinel, and is therefore one of the distinguishing marks in testing the nature of the red gems. The red tourmaline is also strangely liable to internal fibres, hollow threads, clouds, and longitudinal streaks, and sometimes presents an appearance similar to that of the ruby. The red sapphire is also distinguished from the other varieties by being decidedly heavier, and also by being softer than the deep-blue. Ceylon is famous for the abundance of blue sapphires, while the red variety is comparatively rare. In Burmah, however, the red variety is the most abundant and of the finest hue. The Ceylon rubies are regarded as inferior in tint to those found in Ava and Pegu of the Burmese Empire; but they are less inclined to be opalescent, and are therefore more brilliant. The violet tinge of the Ceylon rubies lessens their beauty when viewed by daylight; but it disappears in a great measure by artificial light, and the hue then becomes of a fine prismatic red, accompanied by the most vivid lustre; therefore we may say in general terms that the Burmese rubies are the most beautiful by daylight, and that the Ceylonese are superior by night.