362 PHILOSOPHICAL TRANSACTIONS. [ANNO 1798. 



described as the 3d modification of the corundum crystal, in the analytical descrip- 

 tion which follows. The chatoyant or play of light, on these dark crystals, is very 

 remarkable : some are of a bright copper colour ; others exhibit the accident of re- 

 flection of light, which, in a polished state, gives varieties to the cat's eye, star- 

 stone, sun-stone, &c ; which, as yet, are classed from such accident, without 

 strict attention to their nature, which is various, and in general has not been 

 ascertained. 



These are the circumstances connected with the strata worth mentioning. The 

 examination of corundum on which our present knowledge rests, is nearly that 

 which an Indian mineralogist might derive of the history of feldspar, from a lump 

 of Aberdeen granite, out of 1 or 2 different quarries. He might ascertain a few 

 modifications of the crystal of feldspar, its fracture and matrix ; but he would have 

 no knowledge of the purest or more beautiful sorts which other quarries produce, in 

 Scotland, at Baveno, at St. Gothard, and in Auvergne. I therefore think it 

 essential to mention, that corundum, under circumstances favourable to its crystal- 

 lization, becomes glassy in its fracture, and of various colours. I have not only 

 observed, in crystals of corundum, specks of a fine ruby colour; but I have frag- 

 ments of crystals, in texture and every respect like the colourless corundum, of a 

 fine red colour. It is certain that we obtain from India, corundum which may pass 

 for rubies. I have sent to India some of the corundum with small ruby specks, 

 which were not sufficiently distinct or large either for measurement or analysis, in 

 hopes of being enabled to ascertain correctly the form of Salam rubies found in co- 

 rundum ; in the mean time, I have the corundum of a fine red colour. Looking 

 over some polished rubies from India, I selected one which appeared laminated like 

 corundum, and had also the chatoyant or play of light on its laminae, which formed 

 an angle in the stone. The lapidary called it an oriental ruby. I altered the form 

 of the cutting, so fortunately, that the reflected rays formed a perfect star; a 

 phenomenon I had observed in the sapphire, and expected in corundum, but not 

 in the octoedral ruby. The specific gravity of 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, that Bergman has stated the variety of specific gravity in gems 

 to be so great, as to leave no certain rule of judging by it of the species. He ob- 

 served, that the topaz generally prevails in weight, being from 3. 460 to 4.5 60 ; 

 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, " Analysi crystallorum, tarn ejusdem quam diversae 

 * De Terra Gemmarum. Bergm. Opusc. vol. 2, p. 104. — Orig. 



