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The Magnificent Turquoise Stone


This stone is of very ancient origin, and many old turquoise deposits, now empty, have been discovered in various places. History records a magnificent turquoise being offered in Russia for about £800 a few centuries ago, which is a very high price for these comparatively common stones. 

The turquoise is a pseudomorph. In colour it is blue or greenish-blue, sometimes opaque, varying between that and feeble translucency, though it should be said that in all forms, even those considered opaque, a thin cutting of the stone appears almost transparent, so that the usual classing of it among the opaque stones must be done with this reservation. 

turquoise stone
In composition it contains about 20 per cent. of water, about a third of its substance being phosphoric acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina, with small quantities of iron in the form of variously coloured oxides, with oxide of manganese. 

The great proportion of water, which it seems to take up during formation, is mostly obtained in the cavities of weathered and moisture-decomposing rocks. Its average formula may be said to be Al2O3P2O5 + 5H2O, and sometimes Al2O3 FeOP2O5 + 5H2O. 

It must therefore follow that when the stone is heated, this water will separate and be given off in steam, which is found to be the case. 
The water comes off rapidly, the colour of the stone altering meanwhile from its blue or blue-green to brown. 
If the heat is continued sufficiently long, this brown will deepen to black, while the flame is turned green. 
This is one of the tests for turquoise, but as the stone is destroyed in the process, the experiment should be made on a splinter from it. 

Owing to the presence of phosphorus in bones, it is not uncommon to find, in certain caves which have been the resort of wild animals, or into which animals have fallen, that bones in time become subjected to the oozing and moisture of their surroundings; alumina, as well as the oxides of copper, manganese and iron, are often washed across and over these bones lying on the cave floor, so that in time, this silt acts on the substance of the bones, forming a variety of turquoise of exactly the same composition as that just described, and of the same colour. 

So that around the bones there eventually appears a beautiful turquoise casing; the bone centre is also coloured like its casing, though not entirely losing its bony characteristics, so that it really forms a kind of ossified turquoise, surrounded by real turquoise, and this is called the "bone turquoise" or "odontolite." 

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