402 SYSTEMATIC DESCRIPTIOxN OF PRECIOUS STONES 



the conchoidal fracture characteristic of glass may be detected. Moreover, unless the paste 

 is made with the greatest possible care, it is almost certain to contain air-bubbles, which 

 are never present in genuine stones. A few other substances, sometimes substituted for 

 turquoise, will now be considered below. 



BONE-TURQUOISE. 



Bone-turquoise is also known by various other names, such as tooth-turquoise, 

 occidental turquoise, turquoise de la nouvelle roche, odontolite, and fossil turquoise. The 

 name is applied to the bones, and especially to the teeth, of certain extinct vertebrates, 

 such as the Mastodon and Dinotherium, which during the process of fossilisation have taken 

 up phosphate of iron, the so-called blue iron-earth, and so acquired a fine sky-blue colour, 

 or by taking up copper salts have assumed a green colour. The latter case is rarely met 

 with, and as bone turquoise of a green colour is not prized it need not be considered further. 



These blue-coloured fossil teeth and bones are found at various localities, and are 

 especially abundant in the Miocene beds of Simorre, Auch, and other places in the depart- 

 ment Gers (province Gascogne) in France, where they are in most cases the remains of 

 Mastodon angustidens. At Simorre the deposit was for a short time even systematically 

 mined. When first taken out of the ground the teeth are of a dingy greyish-blue colour, 

 which changes when they are heated to a fine sky-blue. The teeth of the mammoth, so 

 often found in Siberia, are also sometimes of a fine blue colour. 



Bone-turquoise, like true turquoise, is frequently cut en cabochon, and, though much 

 less valuable than the latter, commands a fair price, especially in fine pieces of considerable 

 size. Its colour by daylight is scarcely distinguishable from that of true turquoise, but in 

 candlelight it assumes a dull grey hue, and when immersed in alcohol or water gradually 

 fades. On cut and polished surfaces paler stripes on a darker background may frequently 

 be detected, an appearance which is due to the structure of the tooth. This structure can 

 be clearly made out by examining thin sections of the material under the microscope ; and at 

 the same time it will be seen that the colouring matter is collected in small tubular cavities. 

 Specimens showing dendritic markings, that is to say, moss-like patches of a brown or black 

 colour, are met with not infrequently ; they are much less valuable than material which is 

 not so marked. 



Bone-turquoise may be easily distinguished from the true mineral turquoise. In the 

 first place the former contains up to 11 per cent, of calcium carbonate, hence when a small 

 fragment of bone-turquoise is placed in hydrochloric acid, or a drop of acid placed on a 

 large piece of the same substance, there is a brisk effervescence. Moreover, when heated, 

 a distinct smell of burning becomes perceptible, owing to the presence of organic matter in 

 the substance. When rubbed it becomes electrified and retains its charge for a long time. 

 It is less hard than true turquoise, and is not, therefore, susceptible of as high a degree of 

 polish. The specific gravity, on the other hand, is greater, ranging from 3 to 3 J, so that a 

 piece of bone-turquoise sinks in liquid No. 3 (sp. gi-. = 3-0), while mineral turquoise floats. 

 The dull grey colour assumed by bone-turquoise in artificial light also serves to distinguish 

 it from mineral turquoise. 



An imitation of bone-turquoise may be produced by allowing calcined ivory to remain 

 for a week immersed in a warm solution of copper sulphate, to which excess of ammonia 

 has been added. After this treatment the ivory acquires the same fine blue colour as 

 naturally occurring bone-turquoise. 



