THE MICAS, ASBESTOS, AND GEMS 161 
fibres 2—3 feet long ; it is more resistant to acid, but is weaker 
than chrysotile asbestos, Anthophyllite asbestos (from a 
rhombic amphibole) is a brittle variety quarried in Georgia 
(U.S.A) for use in building tiles. A. M. Bateman (Econ. 
Geol., xviii, 1923, pp. 663-80) has described an Arizona 
asbestos due to replacement of thin bands of serpentine 
which had been formed along earthy layers in a limestone 
by magmatic water from an adjacent diabase. This ser- 
pentine was not an’ altered ultra-basic rock. 
The value of asbestos varies greatly ; in 1925 the average 
of all grades was about £7 per ton, while the highest quality 
Was worth about £100 per ton. 
MoONAZITE 
Monazite (Ce(LaDi)PO,; sp. gr., 5-2; hardness, 54) is a 
phosphate of the rare earths cerium, lanthanum, and didy- 
mium, and yields the thorium oxide used for gas mantles. 
Monazite occurs in yellow grains as an accessory constituent 
of gneiss, pegmatite, and many granites. It is obtained from 
alluvial deposits and mainly from sea beaches in Southern 
India and Brazil. The Indian is the richer and yields about 
8 per cent. of thorium oxide. Owing to the smaller use of 
gas as an illuminant the price of monazite sand has fallen 
to about 3d. per Ib. and the production in India from 2000 
tons in 1918-19 to an average of 300 tons; the output from 
Brazil, which was 7000 tons per annum, has stopped. 
Tur Gems 
Tue Diamonp—The gems are mostly common materials, 
such as carbon, alumina, and earthy silicates, crystallized 
by contact metamorphism, or superheated steam, or acids 
under pneumatolytic conditions. The diamond is first in 
scientific and historic interest. It was found to be a crystal- 
line form of carbon, as its ignition by a burning glass produced 
only carbon dioxide. The diamond is the hardest known 
natural substance. Its hardness led to the legend that it 
1s indestructible unless treated with goat’s blood. The chief 
supplies have come from the interior of South Africa, 150 
million carats! from Brazil, 15 million carats. from India. 
Th 
€ metri 
standard sj Tieunaiof 
200 milli 
2 since 1913 ; th 0 milligrams = 3 . 
; the former carat has been th 
s 34 grains e accepted