MR. J. T. HOTBLACK ON PRECIOUS STONES. 
19 
Spinel. 
The next of the alumina stones is the spinel, which has about 
70.0 of alumina, and nearly all the rest magnesia, it is reckoned 
eight in the scale of hardness; its S.G. 3.5 to 4.8. This stone is 
called by its proper name when white, blue, green, or dark red, but 
when a lighter red it is the balas ruby, and this is the form in 
which it is usually met with ; in fact, most of the rubies which 
I have seen in jewellery have been only balas rubies, their lustre is 
very inferior to that of the true ruby. 
Topaz. 
The next and last of the stones, with more alumina than 
anything else in their composition, is the topaz, it has from 
50.0 to GO.O of alumina, 25.0 to 35.0 silica, and about 15.0 llorine. 
This is the first stone, reckoning down from the diamond, with any 
considerable proportion of silica in its composition, it is reckoned 
eight, or nearly eight, in the scale of hardness ; its S.G. is 3.4 to 
3.G. This stone is called by its proper name when white, yellow, 
pink, orange, cinnamon, or blue, when sea-green it is an aqua 
marine, when light blue it is called Brazilian sapphire, when 
greenish yellow Brazilian chrysolite, when pinky red Brazilian 
ruby, but in the latter cases has frequently been artificially 
coloured. The topaz is of very small value as a jewel ; still, 
I believe that most of the stones sold as topaz, even when of the 
true topaz colour, i.e., pale yellow, are only yellow quartz, and 
a great deal of aqua marine is but sea-green quartz. 
Emerald or Beryl. 
Perhaps the next in hardness is the emerald or beryl, as it is 
variously called, according to its colour; it is reckoned 7.5 to 8.0 
in the scale of hardness, its S.G. is 2.65 to 2.75, and its composition 
is about 68.0 silica, 16 alumina, and 14 glucina or berylla. This 
stone, when a fine green colour, is the true emerald, though not 
so valuable as the “ Oriental ” or corundum emerald, when pale 
sea-green it is an aqua marine, and when white, blue, yellow, or 
pink is called beryl. It is sometimes found in crystals of immense 
size. There was one from America at the 1851 Exhibition 
weighing 78 lbs., and there has been one reported as found in 
c 2 
