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Ch. 7: Turquoise

Ch. 7: Corundum: Sapphire Rubies etc. Page of 149 Ch. 7: Turquoise Text size:minus plus Restore normal size   Mail page  Print this page
       
     
 
70
PRECIOUS STONES.
 
 

 
 
indeed the same substance, but in a vitreous or non-crystalline state. The specific gravity of this lime spinel, which has been sold for blue sapphire, is lower than that of sapphire or even than that of blue spinel.
Turquoise.
The turquoise acquired its name from having been imported into Western Europe by way of Turkey. The best specimens come from the district of Nishapur, in the Persian province of Khor-assan, where the gem occurs in a porphyritic rock. The hardness of the stone is nearly 6, and its specific gravity 2'75. There is a peculiar quality in the colour of the best turquoises, which is partly dependent upon the delicate hue of its blue, with which a slight infusion of green is mingled, and partly upon the faint translucency of the stone. For turquoise is indeed not opaque, thin splinter.; transmitting light easily.
It is very probable that turquoise was described by Pliny under the three names of callais, callaina, and callaica. Turquoise is often now called callaite, while an allied mineral from a Celtic grave near Mane-er H'roek in Lockmariaquer, and now preserved in the Museum of the Polymathic Society of Morbihan, has been called callais and callainite, but has lately been proved identical with the variscite of Breithaupt, a mineral described in 1837.
The true turquoise, which shows various hues and tones of blue, greenish blue, and bluish green, is not to be confounded with the blue fossil turquoise, or odontolite, which is in fact fossil ivory, generally of Mastodon teeth. The true turquoise owes its colour to phosphate of copper, and its powder becomes dark blue when moistened with strong ammonia. Odontolite is coloured by phosphate of iron, is more opaque and heavier than turquoise, and much softer, and shows its bony structure under the microscope, Turquoise often becomes green by age ; this change is frequently noticeable in the turquoise cameos of the Italian cinque-cento.
 
 

 
       
Ch. 7: Corundum: Sapphire Rubies etc. Page of 149 Ch. 7: Turquoise
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