142 
EARTHS. SILICEOUS. 42* Gemma, 
ORDER V. SILICEOUS. 
Conftfiing ‘principally of Silica: hard. 
42. GEMMA. Confifting of filica and a larger 
proportion of alumina, with Ibmetimes a little 
carbonate of lime, and oxyde of iron ; meagre to 
the touch, of a high internal luftre, very rarely 
opake or fubopake, never hardifli or foft, break- 
ing into indeterminate fragments, parafitical, 
fhining in the dark, attrafting light bodies 
when heated by friftion: not melting with 
alcalies. 
Rubinus, Very hard, ponderous, red, of a foliated texture, which 
in a contrary diredlion is conchoidal, not melting or 
lofing its colour in the fire. 
Alumen Gemma Rubinus. Sj/fi. nat. xii. 3 p. loz. ». 6. 
Gemma pellueidiflima. Wall.fjst, t. p. 235, n. 2. 
Oriental Ruby. KU-’wan mintr. l.p. 250. 
True Rubv. SchmeiJJir miner. 1 . p. 60. 
Perfeft Corundum. Thomfon chem, 3.#- 505. 
Found in Bratil and the Eafl hdiesf principally in the kingdoms 
of Peru and Ceylon, and is, except the Diamond, the moft 
precious of all the gems : colour carmine red, fometimes 
verging to violet, or between carmine and hyacinth red, 
fometimes red and white or red jind blue, or orange-red : is 
found in angular pieces, in fmjill pebbles, or in regular 6- 
fided pyramids joined to and ojipofed bafe to bafe : feldom 
exceeding an inch in fize; when finely powdered, melting 
• wiih borax, though with difficulty into a greenilh glafs: 
fpecific gravity from 3,76. 104,283. contains alumina 40, 
filica 39. carbonate of lijne 9. ox)de of iron lO. 
Bergman. 
Sapphirus,^ Very hard, foinewhat ponderous, blue, making a white 
ftreak, of a Ilightly incurved lamellar texture, not 
fufible but lofing its colour in a ftronghcat. 
Alum, lapid. pellucid. Syst, nat. xii. 3. p. 103. n. 6.C, 
Gemma pellueidiflima. Wall. Jytt. min. x.p.-iyj. n, 3, 
Oriental Sapphire. Kirnti an miner, i.p. 252. 
Perfeft Corundum. Thomfon chem. 3. p. 505. 
Sapphire. Schmeijfer min, i.p. 58. 
