On the Corundum Stone from Afia. ^Jj 



frix being mixed with a red and white fparry fubftance and mica, is generally called red 

 granite; but it appears to me of the fame nature as the matrix of corundum from India. 

 The white is more fibrous, and. like cyanite; the red part of it is compa£l and opaque; 

 Other parts appear to foliate, and pure mica is in confiderable patches, and generally adheres 

 to the cryftala. This corundum is of a darker brown, and more irregular on the furface^ 

 than the corundum of the coaft, and often mixed with black iron ore* attraiiable by tlie 

 magnet. 



It is defcribed asthe third modification of the corundum cryftal in the analytical defcrip- 

 tiori which follows. The chatoyant, or play of light, on thefe dark cryftals, is very remark- 

 able : fome are of a bright copper colour; others exhibit the accident of refle£tion of light, 

 which, in' a polifhed ftate, gives varieties to the cat's-eye, ftar-ftone, fun-ftone, &c; which as 

 yet are clafled from fuch accident without ftridl attention to their nature, which is various, 

 and, in general, has not been afcertained., 



Thefe are the circumftances connefted witlrthe ftrata, worth mentioning^ The examina- 

 tion of corundum, on which our prefent knowledge rells, is nearly that which an India mi- 

 neralogift might derive of the hiftory of feldfpar from a lump of Aberdeen granite out of 

 fine or two different quarries. He might afcertaia a few modifications ^f the cryftal of feld- 

 fpar, its frafture, and matrix ; but he would have no knowledge of the pureft or more beau- 

 tiful forts, which other quarries produce in Scotland, at Bavcrno, at St. Gotliard, and Au- 

 vergne. I therefore think it effential to mention that corundum, under circumftances fa- 

 vourable to its cryftallization, becomes glafly in its frafture, and of various colours. I have 

 not only obferved in cryftals of corundum fpecks of a fine ruby colour, but I have fragments 

 of cryftals in texture and every refpe£t like the colourlefs corundum, of a fine red colour. 

 it is certain that we obtain from India, corundum which may pafs for rubies. I have fent 

 to India fome of the corundum with fmall ruby fpecks, which were not fufficiently diftin£l 

 or large, either for meafurement or analyfis, in hopes of being enabled to afcertaiu correiHy 

 the form of Balam rubies found in corundum ; in 'the mean time, I have the corundum of 

 a fine red colour. Looking over fome poliflied rubies from India, I feleiSled one which ap- 

 peared laminated like corundum, and had ajfb the chfitoyant, or play of light, ,on its laminae, 

 which formed an angle in the ftone. The lapidary called it an oriental ruby. I altered the 

 form of the cutting fo fortunately that the reflefted rays formed a perfe£i: ftar; a phjenomenoa 

 I had obferved in the fapphire, and expelled in corundum, but not in the oftoedral ruby. 

 The fpecific gravity of tliis ftone being 4, j 66, confirmed my opinion- that it is one of the 

 Salai7i rubies, fo much efteemed by tlie natives on the coaft or peninfula of India, which are 

 found in the corundum vein. The_fpecific gravitj^ of a colourlefs fapphire very fittle lefs 

 opaque than corundum, forming a'fo.a perfe<3 ftar, yas 4^000 ; that of a deep, blue fapphire, . 

 and of a ftar-ftone, 4,035 ; all which J conneft with the corundum ; the fpecific gravity of a 



. , ■ ■• « 



* A fmall group, confifting of three or four cftoedral cryftals, prefents the lead common variety of thi» • 



kind of iron ore ; the edges of the oftoedra being replaced by planes, which altnoft cover the triangular planes, 



Rome de i'ljk. Criftallcg. vol. IV. plate 4, fig. 6g, 



diftina 



