CORUNDUM (SAPPHIRE): CHARACTERS 283 



out of a parti-coloured sapphire in such a way that the head of the figure is colourless, the 

 legs yellow, and the body pale blue. 



Every shade of blue, from the palest to the darkest, is represented amongst sapphires. 

 The very dark shade of blue, closely approaching black, is described as inky ; very pale 

 " feminine " sapphires are sometimes described as " water-sapphires," while stones of the 

 darkest shade are variously known as " indigo-sapphire," " lynx-sapphire " or " cat-sapphire." 

 So long as the depth of its colour does not interfere with the transparency of a sapphire, the 

 darker it is in colour the more highly is it prized. The colour of sapphires shows as great a 

 variety of tone as of shade. Thus, we have stones of indigo-blue, Berlin-blue, smalt-blue, 

 cornflower-blue, greyish-blue, and greenish-blue, the last being specially common. The 

 most admired tone of colour for a sapphire is an intense comflower-blue. A really fine 

 sapphire will combine with this colour a beautiful velvety lustre ; the latter character,, 

 though occasionally seen, is by no means common. A fine blue crystal of sapphire is shown 

 in Plate I., Fig. 7, and a faceted stone in Fig. 8 of the same plate. 



The blue of the sapphire is always more or less tinged with green, this being very 

 noticeable when one looks through the stone in certain directions. Like the ruby the 

 sapphire is distinctly dichroic, the phenomenon being very marked in dark coloured stones 

 but scarcely noticeable in stones of a light shade of colour. Looked at in the direction of 

 the optic axis, that is to say, along the line joining the apices of the hexagonal bipyramid or 

 perpendicular to the terminal basal planes, a crystal of sapphire appears of a pure blue 

 colour, more or less intense or inclined to violet, according to the particular character of the 

 stone. In a direction perpendicular to this the stone appears paler, and its blue colour is 

 distinctly tinged with green ; observed in intermediate directions the sapphire will appear 

 of intermediate tints. The dichroisni of the sapphires of Siam, which have lately come into 

 the market in large numbers, as well as of those from I^e Puy, in Auvergne, and from some 

 other localities, is especially well marked. 



If the light passing through a crystal of sapphire in the direction of its optic axis be 

 received in a dichroscope, the two images formed by the instrument will be identical in 

 colour — either pure blue or blue tinged with violet — and the colour will remain unchanged 

 when the stone or the instrument is rotated. If examined in the direction perpendicular to 

 this, the two images will, as a rule, be coloured differently. In the position in which the 

 greatest difference in colour exists, the one will be of a pure dark blue and the other usually 

 of a paler greenish-blue, but sometimes of a yellowish-green. 



It follows from these facts that the pure blue colour of a sapphire crystal is best 

 displayed in the cut stone when the table of the latter is perpendicular to the optic axis, 

 and parallel to the basal planes, of the crystal. It will be remembered that it is 

 advantageous for the same reason to cut crystals of ruby also in this manner. 



The appearance of sapphire in artificial illumination varies in different specimens. In 

 some no difference in colour can be detected, in others the colour becomes darker, or it 

 may change to I'eddish, purple, or violet. The latter change in colour is rare, and stones 

 which show it are valuable on this account. 



While the colour of the ruby remains unaltered after the stone has been exposed to a 

 strong red-heat, that of the sapphire under similar conditions disappears, although in other 

 respects the stone remains unchanged. When exposed to very high temperatures, however, 

 the sapphire, like the ruby, becomes grey and cloudy. The decolorisation of the sapphire 

 does not take place with equal facility in all stones. Indian sapphires lose their colour most 

 easily ; and there are stones the colour of which it is impossible to completely destroy. 

 Fi'om the fact that sapphires can be decolorised by heat, it has been argued that their 



