26 
MR. J. T. HOTBLACK ON PRECIOUS STONES. 
never found except in very small pieces. It is also the softest of 
all the gem stones, being 6 only in hardness, with a S.G. of 2.6 to 
3.0. I have notes of the following compositions : — 
Alumina 
Silesia. 
44.50 
Persia. 
47.45 
46.90 
Phosphoric Acid 
30.90 
27.35 
32.60 
Water ... 
19.00 
18.20 
20.50 
Protoxide of Copper ... 
3.75 
2.00 
— 
Phosphate of Lime 
— 
3.40 
— 
It is found white and green, and probably in other colours 
besides the orthodox turquoise blue. Stones of this last colour are 
the only ones that have any commercial value, and they come 
almost entirely from Persia. Good stones large enough to set in 
single stone rings are readily worth from £10 to £40 each, and 
larger stones of a good colour are so extremely rare that they fetch 
almost fabulous prices. Turquoise are always cut en cabochon. 
Turquoise are found in groups of small nodules in sandstone, also 
in veins about a tenth of an inch in thickness, crossways of the 
bedding of the sandstone. 
There is a fossil bone, “ odontolite,” of turquoise colour, which 
is sometimes sold for the true stone, but it is generally of an 
inky blue colour, which is never seen in the real turquoise, and its 
texture often shows traces of its organic origin. Many artificial 
turquoise are made, and very successfully for colour, but they may 
be easily distinguished from the real stone by its greater lustre and 
better gloss. 
Having finished (with the exception of the quartz family which 
I propose to treat separately and last) all the stones generally used 
in jewellery, I think it necessary to treat of three out of the four 
other principal substances used for jewellery. The one I shall not 
treat of is the pearl, because being a substance of present animal 
growth it has no interest to the mineralogist. 
Malachite 
is really a metal, being a form of copper ore. Its hardness is but 
3.5, its S.G. 3.7 to 4.0, and I find its composition stated as under: 
Protoxide of Copper 
70.5 
72.2 
Carbonic Acid 
18.0 
18.5 
Water 
11.5 
9.3 
