«48 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



may be, is distributed throughout the substance of the stone with perfect uniformity and 

 without patchiness. When heated the mineral retains its colour, or if this be altered, it 

 rehurns to the original shade when cool again. Pure calcium-aluminium garnet is perfectly 

 colourless ; this white, so-called leuco-garnet is, however, never cut as a gem. The 

 commonest colour for garnet is red. Every shade between the palest and one dark enough 

 to be almost black is represented ; while the most diverse tones of red are met with, the 

 most usual being tinged with brown, yellow, or violet. Red garnet, the colour-variety 

 which is almost exclusively used for gems, was formerly one of the stones to which the name 

 carbuncle was applied. It is probable that the term carbuncle formerly included all red 

 stones, and not only the ruby, to which the application of the term is preferably limited at 

 the present time. The name of the mineral is said to have been derived from the colour of 

 the gem-varieties, i-cd garnets having been compared to the flowers and seeds of the 

 pomegranate-tree. 



Green garnet, the colour of which is comparable to a certain extent to that of the 

 emerald, though usually more yellowish or brownish, is also cut as a gem, but much less 

 frequently. The finest emerald-green variety, the calcium-chromium garnet, or uvarovite, 

 unfortunately occurs in such small and imperfectly transparent crystals that it is useless as 

 a gem. 



The colour of garnet is due to iron, and to a lesser extent to manganese and chromium. 

 Iron is responsible for the reds, yellowish-greens, and the connnonly occurring yellows and 

 browns, while the colour of the emerald-green garnet, uvarovite, is due to chromium. 



Black garnets also occur, their colour being also due to the presence of iron ; they are 

 occasionally used in mourning jewellery. Blue is a colour conspicuous by its complete 

 absence in the garnet grouj). The subject of colour will be further considered when the 

 different varieties of garnet are dealt with. 



As usual, in all minerals, different specimens of garnet show different degrees of 

 transparency; most crystals are turbid and opaque; but amongst all varieties, numerous 

 specimens of perfect clearness and transparency, sometimes combined with the deepest and 

 darkest shade of colour, are to be found, and it is these alone which are cut as gems. 



The natural faces of crystals vary in brilliancy, the lustre being sometimes very strong, 

 at other times more feeble, owing to the roughness of the surface and to other causes. The 

 freshly fractured surface of a transparent stone is, however, always brilliant, and by grinding 

 and polishing the lustre is still further increased, cut stones being very brilliant. The 

 lustre of garnet is the ordinary vitreous kind, so strongly inclined, however, to resinous 

 lustre that some garnets closely resemble a piece of resin in external appearance. 



Garnet, being a cubic mineral, is optically isotropic, that is to say, it is singly 

 refracting ; anomalous double refraction is to be observed in a few rare cases, but scarcely 

 ever in perfectly clear and transparent stones such as are cut as gems. The mineral has a 

 somewhat high index of refraction ; this, however, like the other physical characters, varies 

 with the chemical composition. The index of refraction for red light ranges from 1-74 to 

 1-79 in the different varieties. The dispersion is almost always small; indeed, the only 

 variety which shows any very appreciable play of prismatic colours is the green calcium-iron 

 garnet known as demantoid. The value of the garnet as a precious stone depends upon its 

 strong lustre and the depth and fulness of its colour. 



In some cases when a candle-flame is observed through a stone, a four- or six-rayed 

 star of light, similar to that shown by a star-sapphire, is seen. This appearance, however, 

 is rare and does not add to the beauty of the stone as a gem, nor to its value as a precious 

 stone. 



