TURQUOISE 391 



however, that in former times turquoise of a green colour was in certain cases, for example 

 by the ancient Mexicans, thought as much of as the blue variety, and we shall see later on 

 that at the present day the natives of these regions frequently wear green turquoise in 

 preference to blue. 



As a rule, the turquoise is of one uniform shade of colour over its whole surface. In 

 the case, however, of stones from certain localities, especially the Sinai Peninsula, there is 

 sometimes visible a network of fine streaks of a paler shade of colour, which in cut stones 

 shows up sharply against the deeper-coloured background. It is characteristic of true 

 turquoise that its blue colour is just as beautiful by candlelight as by daylight, whereas 

 other blue substances resembling turquoise appear of a dingy grey colour. 



The colour of some turquoise is very unstable. Many stones, for example, from the 

 Meghara valley in the Sinai Peninsula and from New Mexico begin to grow dull and pale 

 directly they are taken out of the mine, and after a short time their colour completely 

 disappears. It is stated as a general rule that the blue colour of turquoise is unstable and 

 is gradually bleached by sunlight, the blue at the same time assuming a greenish hue. 

 This is not always the case, however, and many turquoises retain their colour unaltered for 

 a very long period. Sir Richard Burton testifies to having seen set in the musket of a 

 Bedouin a very fine blue stone which, in spite of its exposure to sun, wind and weather for 

 at least fifty years, had retained all its beauty of colour. The colour of turquoise is also 

 said to be easily affected by perspiration from the body. 



The original fine colour of a stone which has been bleached by wear or by prolonged 

 exposure to sunlight can sometimes be restored by immersing it in ammonia, or by the appli- 

 cation of grease ; even, it is said, by wearing it in such a way that it comes in contact with the 

 natural grease of the hand. The restoration is not permanent, however, the blue soon 

 disappearing, so that it is advisable to guard against the possibility of fraud in this 

 direction. Since the alteration in colour usually proceeds gradually from without inwards, 

 it is possible to improve the appearance of a stone by repolishing ; the operation would, of 

 course, need repeating from time to time. 



By the weathering of the mother-rock turquoi.se completely loses its colour and lustre. 

 Thus rough specimens are often met with, the centre portion of which is of a fine blue 

 colour, but is enclosed by an outer layer of dull white weathered material, which must be 

 removed before the stone is cut. Sometimes the weathering process has proceeded so far 

 that the whole mass is altered into a loose, crumbly material in which there may be found 

 here and there grains of blue turquoise still unchanged. 



Since the beauty and value of the turquoise depends almost entirely on its colour, 

 attempts have been made to improve stones which are lacking in this respect by artificial 

 means. A certain amount of success has attended these efforts, the method usually adopted 

 being to impregnate the stone with Berlin-blue after the manner described in detail below 

 under agate. The artificial colouring matter does not penetrate the stone deeply and 

 the coloured layer can be scratched off with a knife. Moreover, stones so treated 

 appear of a dingy grey colour by candlelight, and when immersed in ammonia either 

 become green or lose their colour altogether, which is not the case with stones of a 

 natural colour. 



The specific gravity of turquoise is rather variable, values ranging from 2 6 to 2'8 

 having been observed. It is the least hard of any of the valuable precious stones ; its 

 hardness, which is the same as that of felspar, being represented by 6 on the scale. 

 Turquoise is therefore easily scratched by quartz or by a file, but is just hard enough to 

 scratch ordinary window-glass. On account of its softness it requires special care when 



