390 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



green. It is usually soluble both in hydrochloric acid and in nitric acid, but specimens 

 from different localities behave somewhat differently in this respect, some being unattacked 

 by either of these acids. 



It has already been stated, that turquoise has not hitherto been found in crystals. It 

 occurs as irregular masses, completely or partly filling cracks and crevices and other cavities 

 in the mother-rock. When a rock cavity is completely filled up, the turquoise usually 

 takes the form of a plate, the thickness of which is rarely more than a few millimetres, 

 while its area may be considerable. When, on the other hand, the cavity is only partially 

 filled, the tui-quoise forms a lining of greater or less thickness to its walls, the siitface of the 

 lining layer of turquoise being frequently mammillated, botryoidal, or stalactitic. 



In accordance with the absence of any definite crystalline form, there is a complete 

 absence of cleavage in the mass. The fracture is sub-conchoidal to uneven : a clean, 

 fractured surface shows but little brilliancy of lustre, and has usually a wax-like aspect, 

 though at times the lustre may incline to the glassy or vitreous type. The lustre is greater 

 when the stone is cut and polished, but is never very brilliant, the beauty of the turquoise 

 depending for the most part upon its colour alone. The mineral is opaque, except in the 

 thinnest of splinters through which a certain amount of light only is transmitted. 



The naked-eye appearance of a freshly fractured, or cut and polished, surface of 

 turquoise suggests a perfect continuity of structure. If, however, a thin section of the 

 mineral be examined under the microscope, it is found to be built up of innumerable grains 

 of irregular form arranged in an irregular manner. An examination of these grains in 

 polarised light proves them to be doubly refracting, which demonstrates the fact that 

 turquoise, in spite of the absence of any external crystalline form, is not amorphous, but is 

 a compact aggregate of microscopically small crystalline individuals. In examining thin 

 sections of turquoise under the microscope, less transparent portions with circular outlines 

 are sometimes observed ; such appearances are probably due to the beginning of weathering 

 n the turquoise substance. Small foreign bodies, which may possibly be chalcedony, are 

 also visible sometimes under the microscope. 



Turquoise is either green or blue, the former colour being much more frequently seen 

 than the latter. The colour in both cases is due to the presence of copper phosphate, and 

 probably also of iron phosphate, in small amount, intermixed with the colourless aluminium 

 phosphate, which constitutes the greater proportion of the substance of the stone. In 

 examiningfsections under the microscope it is only rarely, especially in blue Persian turquoise, 

 that the pigment is seen to be located in definite strings or in cloudy patches with 

 ill-defined boundaries. In most cases the colouring matter is in an extremely fine state of 

 division, and is uniformly distributed throughout the turquoise substance. Thin sections 

 of the mineral are almost colourless, perhaps faintly yellow ; the blue or green colour 

 appears only in slices of some thickness. It is not at all unusual, however, for very thin 

 sections of a deeply-coloured and almost opaque mineral to appear colourless and trans- 

 parent. 



The colour of turquoise ranges from sky-blue to mountain-green, the latter being not 

 a pure green, but a green containing both grey and blue tones. Turquoise of a very intense 

 and deep colour rarely occurs, but pale shades in great variety and forming a complete 

 series from blue to green are to be met with. Of all these shades, the pure sky-blue of the 

 deepest possible shade is most prized, and it is only turquoises of this colour which in 

 Europe and the East are valued. The more the colour of a turquoise inclines towards 

 green the less valuable does it become, and specimens of a distinct green colour are 

 used as gems nowhere in the Old World, except in some parts of Arabia. It appears, 



