1911 Encyclopædia Britannica/Sapphire

From Wikisource
Jump to: navigation, search

SAPPHIRE,[1] a blue transparent variety of corundum, or native alumina, much valued as a gem-stone. It is essentially the same mineral as ruby, from which it differs chiefly in colour. The colour of the normal sapphire varies from the palest blue to deep indigo, the most esteemed tint being that of the blue cornflower. Many of the crystals are parti-coloured, the blue being distributed in patches in a colourless or yellow stone; but by skilful cutting, the deep-coloured portion may be caused to impart colour to the entire gem. As the sapphire crystallizes in the hexagonal 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 tourmaline and in iolite — stones sometimes mistaken for sapphire — the dichroism is much more distinct. The blue colour in sapphire has been variously referred to the presence of oxides of chromium, iron or titanium, whilst an organic origin has also been suggested. On exposure to a high temperature, the sapphire usually loses colour, but, unlike ruby, it does 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, the blue colour of sapphire exposed to the action of radium changes to green and then to yellow.

Under artificial illumination many sapphires appear dark and inky, whilst in some cases the blue changes to a violet, so that the sapphire seems to be transformed to an amethyst. According to lapidaries the hardness of sapphire slightly exceeds that of ruby, and it is also rather denser. Notwithstanding its hardness it has been sometimes engraved as a gem.

Ceylon has for ages been famous for sapphires. They occur, with

many other gem-stones, as pebbles or rolled crystals in alluvial deposits of sand and gravel; the gem-gravel being known locally as illam. The principal localities are Ratnapura, Rakwana in the province of Sabara-Gamawa and Matara. Some of the slightly-cloudy Ceylon sapphires, usually of greyish-blue colour, display when cut with a convex face a chatoyant luminosity, sometimes forming a luminous star of six rays, whence they are called “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 deposits 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 of very dark colour, 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). Sapphires are also found in Kashmir, where they occur, associated with tourmaline, in the Zanskar range, especially near the village of Soomjam. Madagascar yields sapphires generally of very deep colour, occurring as rolled crystals. Sapphire is widely distributed through the gold-bearing drifts of Victoria, New South Wales and Queensland, but the blue colour 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 near Anakie station on the Central railway, to the west of Emerald and east of the Drummond Range. Sapphire occurs also in Tasmania. 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 Corundum 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 discovered as far back as 1865, and have been worked on a large scale. They were originally found in washing for gold. The rolled crystals of sapphire occur, with garnet and other minerals, in glacial deposits, and have probably been derived from dykes 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 rhombonedral 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 and of Le-Puy-en-Velay in France, but the European

stones have no interest as gems.
Although the term sapphire is primarily applied to blue corundum,

it is often used in a general sense so as to include all corundum of gem quality, regardless of colour. Hence clear colourless corundum is known as white sapphire or “leucosapphire.” Such stones have been occasionally cut as lenses for microscopes, being recommended for such use by their high refractivity, weak dispersion and great hardness. White topaz is sometimes called “water-sapphire,” a name which should, however, be restricted to iolite (q.v.). Yellow corundum is not uncommon in Ceylon and is termed yellow sapphire or “oriental topaz,” the prefix “oriental” being often applied to corundum. When of pale yellowish-green colour the sapphire is called “oriental chrysolite,” when greenish-blue “oriental aquamarine,” when of brilliant green colour “oriental emerald,” and when violet “oriental amethyst.” (For figure of crystal of sapphire see

Corundum and for artificial sapphire see Gem, § Artificial.)
The so-called “Hope sapphires” of trade have been shown to be artificial blue spinels, coloured by cobalt.
Sapphirine is a rare mineral, not related to sapphire except in

colour. It is a silicate, containing aluminium, magnesium and iron, brought originally from Greenland, and since found in a rock from

the Vizagapatam district in India.

(F. W. R.*)

  1. Indirectly from Gr. σάπφειρος, but there seems no doubt that this term, like the Hebrew sapir of the Old Testament, was formerly applied to what is now called lapis lazuli; the modern sapphire was probably known as ὺάκινθος (hyacinthus).