Mr. Greville on the 
4*6 
this stone, being 4,166, confirmed my opinion that it is one of 
the Salam rubies, so much esteemed by the natives on the Coast 
or Peninsula of India, which are found in the Corundum vein. 
The specific gravity of a colourless sapphire, very little less 
opaque than Corundum, forming also a perfect star, was 4,000 : 
that of a deep blue sapphire, and of a star-stone, 4,035 ; all 
which I connect with the Corundum ; the specific gravity of a 
distinct crystal of which was 3,950; of a fragment of ruby- 
coloured Corundum, 3,959 ; and of a fragment of Corundum 
with vitreous lustre, 3,954. 
It may be objected to me, that Bergman has stated the va- 
riety of specific gravity in gems to be so great, as to leave no 
certain rule of judging thereby of the species. He observed, 
that the topaz generally prevails in weight, being from 3,460 
to 4,560; the ruby from 3,180 to 4,240 ; then the sapphire, from 
3,650 to 3,940.* But in the preceding page he had said, “ Ana- 
“ lysi crystallorum, tarn ejusdem quam diversae figurae, multum 
<£ lucis scientia expectat. Illae quarum antea compositionem ex- 
« plorare licuit, naturali forma per artem privatae erant” It is 
not, therefore, an hypothesis unworthy of examination which 
I advance, that gems derived from the rectangled octoedra, 
whose specific gravity is above 3,300 to 3,800, will be found 
to be diamonds or octoedral rubies ; and these will be easily 
distinguished from each other, by their lustre and hardness. 
Diamonds, whether red, yellow, blue, or white, being hardest, 
though their specific gravity will be less; viz. from 3,356 
to 3,471, as I found among different diamonds in my collec- 
• De Terra Gemmarum. Bergm. Opusc. Vol. II. p. 104- 
