Properties and Varieties

Although corundum is second to the diamond in point of hardness, it is approached much more closely by the minerals next below it in the scale of hardness, than it approaches the eminent and reserved diamond.

Pure corundum has a high specific gravity ranging from 3.94 to 4.08, and this great density makes the specific gravity test in distinguishing it from other stones both easy and important. The differently coloured varieties have not been proved to vary in this particular. Acids will not attack corundum nor is it fusible before the blowpipe. Some specimens when heated in the dark are beautifully phosphorescent. Corundum, by friction, develops positive electricity, which it retains for some time. The lustre of corundum and its fire approach these qualities in the diamond, but the lustre is vitreous instead of adamantine, although it is very durable. Corundum is optically uniaxial and strongly doubly refracting, but the dispersion produced is slight and it is, therefore, incapable of emitting flashes of prismatic colours like the diamond. Coloured corundum crystals are dichroic and the deeper the colour the more pronounced the dichroism. A constant characteristic of coloured corundum gems is that they are as beautiful by artificial light as by daylight.

There are at least nine varieties of corundum used as gems and familiar to nearly all jewellers; the coloured varieties, other than the red ruby and blue sapphire, are named for the gems of other mineral species that they resemble in colour, only with the distinguishing prefix of "Oriental." The arbitrary names and colours are: Ruby ("Oriental ruby"), red; Sapphire ("Oriental sapphire"), blue; Leuco-sapphire (White sapphire), colourless; "Oriental aquamarine," light bluish-green; "Oriental emerald," green; "Oriental chrysolite," yellowish-green; "Oriental topaz," yellow; "Oriental hyacinth," aurora red; "Oriental amethyst," violet.

The colour-varieties of corundum are found in irregular grains and as crystals embedded in some old crystalline rock, as granite or gneiss. The gem-varieties frequently occur as secondary contact minerals, which contact with a molten igneous rock has developed in limestone. These embedded crystals are frequently liberated by the weathering and uncovering of such rocks, and then the crystals are found in the debris in the beds of streams.

Red corundum is supposed to be identical with the anthrax mentioned by Theophrastus and to have been termed carbuncle during the Middle Ages. The colour-tone of the ruby varies greatly, and the presence of deep, intense tones of red causes the term "masculine" to be applied to a gem, while the paler tints suggest the term "feminine." Rubies range from a delicate pink tint through pale rose red to reddish-white, pure red, carmine red, or blood red. A tinge of blue or violet is frequently discernible in these shades. The desired tone in ruby colour was so aptly compared by the Burmese to the blood of a freshly-killed pigeon that the term "pigeon-blood" is the accepted qualification for the colour of the choicest and costliest ruby gems. The colouring is not always uniform, there sometimes occurring alternate layers of colours and colourless stone; a process of heating usually renders the colour uniform. The ruby does not lose its colour when heated, and hence it is assumed that the colouring matter is not organic, as in that case it would be destroyed, but is probably due to a trace of chromium. A graduated increase of heat will not fracture the stone, which upon cooling becomes white, then green, and finally regains its original red colour. The ruby is dichroic according to the direction in which it is seen, and in cutting it this must be taken into consideration; the table-the largest facet surface-should be aligned with the basal planes of the crystal, in order to exhibit the greatest possible depth of colour. The dichroism of the ruby is one of its certain distinctions from spinel, garnet, and other red stones which crystallise in the cubic system and therefore are but singly refracting.