THE MICAS, ASBESTOS, AND GEMS 167 
the hardness of 64 and a sp. gr. of 3-0. Jadeite, a pyroxene, is 
cream coloured and is highly valued in China; it is found in 
Southern China and Burma; its hardness ranges from 6% to 
7, but it is most conveniently distinguished from nephrite 
by its sp. gr., which is 33. 
Lapis lazuli is a mixed mineral of rich blue colour, and 
consists of an aggregate of hauyne, diopside, amphibole, 
mica, and pyrites. 
Garnets—The garnets include many species, which crystal- 
lize in the cubic system and have a dodecahedral cleavage, 
and are double silicates of alumina, iron or chromium, with 
lime, magnesia, manganese or iron. (Hardness 6% to 7; 
Sp. gr, 3:2 to 4:3.) The chief garnet used as a gem is the 
ruby-red pyrope, a silicate of magnesia and alumina; it is 
common in the South African diamond mines, and is known as 
“the Cape ruby". A brilliant emerald green garnet, andradite, 
from the Urals is known as the * Ural emerald.” Carbuncle 
or almandine is the iron-alumina garnet. Cinnamon-stone, 
of a cinnamon-brown colour, is the lime-alumina garnet, and 
is found in altered limestones, as in Ceylon. A small green 
garnet from the Urals, which is used in sprays of colour and 
called olivine, is the lime-chromium garnet, uvarowite. 
The garnets are generally formed as metamorphic pro- 
ducts, and are common in schists, gneisses, and crystalline 
limestone. 
Olivine in clear transparent varieties is used as a gem 
under the name of peridot. 
Opal is a common cryptocrystalline form of silica. Some 
varieties which are iridescent from the interference effect 
of minute internal surfaces, are of value as gems. The 
chief supply now comes from Cretaceous sandstones in 
Australia. Its origin by replacement is shown by the ex- 
istence of shells and bones composed of precious opal. Some 
opal was formed in the fissures of igneous rocks, where shrink- 
age during cooling has continually strained the veins and pro- 
duced the interference effect. 
The filling of cavities and replacement of irregular nodules 
in a rock by silica produces agate, which often shows its 
formation in successive layers by its banded structure. 
Silica stained of different colours forms the varieties cornelian, 
onyx, jasper, etc.