SAPPHIRE, a blue transparent variety of corundum (q.v.), or native alumina, much valued as a gem-stone. It is essentially the same mineral as ruby, from which it differs chiefly in colour which, normally, varies from palest blue to deep indigo, the most esteemed tint being that of the blue cornflower. Many crystals are parti-coloured, but by skilful cutting, the deep-coloured por tion may be caused to colour the entire gem. As the sapphire crystallizes in the rhombohedral system it is dichroic, but in pale stones this character may not be well marked. In a deep-coloured stone the colour may be resolved, by the dichroscope, into an ultramarine blue and a bluish or yellowish green. In blue tour maline and in iolite—stones sometimes mistaken for sapphire— the dichroism is much more distinct. The blue of sapphire has been referred to the presence of oxides of chromium, iron or titanium, whilst an organic origin has also been suggested. On exposure to high temperature, sapphires usually lose colour, but, unlike rubies, do not regain it on cooling. A. Verneuil succeeded in imparting a sapphire-blue colour to artificial alumina by addition of 1.5% of magnetic oxide of iron and 0.5% of titanic acid (Comptes rendus, Jan. 17, 1910). According to F. Bordas, sapphire exposed to the action of radium changes to green and then to yellow.
Under artificial illumination many sapphires appear dark and inky, and in some cases the blue changes to a violet. In spite of its hardness, which slightly exceeds that of ruby, it has been sometimes engraved as a gem.
Sapphires occur, with many other gem-stones, as pebbles or rolled crystals in the alluvial deposits of sand and gravel of Cey lon, the gem-gravel being known locally as illam. The principal lo calities are Ratnapura, Rakwana and Matara. Some of the slightly-cloudy Ceylon sapphires, usually greyish-blue, display when cut with a convex face a chatoyant luminosity, sometimes forming a luminous star of six rays, or "star-sapphires" (see ASTERIA). The asterism seems due to the presence of microscopic tubular cavities, or to enclosure of crystalline minerals, arranged in a definite system. In 1875 sapphires were discovered in de posits of clay and sand in Battambang (Siam), where they have been worked on a considerable scale; they occur also with rubies in the provinces of Chantabun and Krat. Many of the Siamese
sapphires are very dark, some being so deeply tinted as to appear almost black by reflected light. In Upper Burma sapphires occur in association with rubies, but are much less important (see RUBY), and they are also found in the Zanskar range, Kashmir, especially near the village of Soomjam, associated with tourmaline. Madagascar yields sapphires as rolled crystals generally of very deep colour. They are widely distributed through the gold-bearing drifts of Victoria, New South Wales and Queensland, but the blue of the Australian stones is usually dark, and it is notable that green tints are not infrequent. The Anakie sapphire-fields of Queensland are situated to the west of Emerald and east of the Drummond Range. Coarse sapphire is found in many parts of the United States, and the mineral occurs of gem quality in North Carolina and Montana. The great corundum deposits of Cor undum Hill, Macon county, N.C., have yielded good sapphires, and they are found also at Cowee Creek in the same county. In Montana, sapphires were found in washing for gold in 1865, and have been worked on a large scale. The rolled crystals of sapphire occur, with garnet and other minerals, in glacial de posits, and have probably been derived from dikes of igneous rocks, like andesite and lamprophyre. They display much variety of colour, and exhibit peculiar brilliancy when cut, but are often of pale tints. The principal localities are at Missouri Bar, Ruby Bar and other places near Helena, where they were first worked, and also at Yogo Gulch, near Utica. The Helena crystals are of tabular habit, being composed of the basal pinacoid with a very short hexagonal prism, whilst at Yogo Gulch many of the crystals affect a rhombohedral habit. The Montana sapphires and the matrix have been described by Dr. G. F. Kunz, Professor L. V. Pirsson and Dr. J. H. Pratt (Amer. Jour. Sc., ser. 4 vol. iv., 1897). The sapphire occurs also in Europe, being found in the Iserweise of Bohemia and in the basalt of the Rhine valley, Le-Puy-en Velay in France, and in tholeiite intrusions in the island of Mull, but the European stones have no interest as gems.