142 PARTUS. SILICEOUS. 42. Gemma. 



ORDER V. SILICEOUS, 



ConfijVmg frmct'paUy of Silica: hard, 



42. GEMMA. Confifting of filica and a larger 

 proportion of alumina, with fometimes a little 

 carbonate of lime, and oxyde of ire n: meagre to 

 the touch, of a high interna} kiHre, very rarely 

 opake or fubopake, never hardifh or fofc> break- 

 ing into indeterminate fragments, parufitical, 

 fliining in the dark, attradling light bo<1ies 

 when heated by friclion : not meitipo: with 

 alcalies. 



^it^inau Very hard, pondcrons, red, of 2 foliated textnrc, which 

 in a contrary dirciition is conchoidal, not mehing or 

 lofing its colour in the fire, 



Alumen Gemma Riibinas, Sj/I., tiat. xii. 5 p. iq2, », 6. 



Gemma pellucidillima. Wall.Jyst. \.p 235, «. 2- 



Orientat Ruby. Kir^wan mimr. i./>. 2^0. 



True Rubv. SchnulJJer miner, k />. 60. 



Perfecfl Conindun-i. Thomfon chem. ^.p- 505'. 



Foond in Bra/iJ and the Eaftlridu'. principally in the kingdoms 

 of Psru and Ceylon.^ and is, except the Diamond, the moft 

 precious of all the gems: eolour carmine red, fometimes 

 verging to violer, or between carmine and hyacinth red, 

 lometimes red and white or red .ind blue, or orange-red ; is 

 found in angular pieces, in fmall pebnJes, or in regular 6- 

 fided pyramids joined to and oppofed bafe to bafe : feldom 

 exceeding an inch in fize: when finely powdered, melting 

 wiih borax, though with difficulty into a greenifh glafs : 

 fpeciiic gravity from 3,76, to 4,283. contains alumina 40, 

 filica 39. carbonate of lime 9. ox} de of iron 10* 



Bapphirus, Very hard, fomevvhat ponderous, hVa^y making a white 

 ftreak, of a ilightly incurved lamellar texture, not 

 fufible but lofing its colour in a ftrong heat. 

 Alum, lapid. pellucid. Syst. nat. xii. 3. p» 103. «. 6.c. 

 Gemma pellucidiffima. Wall, fyit.min. i./, 237. n. J, 

 Oriental Sapphire. Kirixian miner, \. p.z^z, 

 PerfeQ; Corundum. 7ho7nfon chem, 3, p, 505, 

 bapphirc. Schmeijfer min, 1,^,58. 



