Mineral Resources Potential - Geothermal Resources

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Gems~ones and Semi-precious Stones 187
to the surface by the basaltic magmas. Most of the world's
emeralds are mined from low-grade carbonaceous schists in
Colombia, South America. In Australia, emeralds occur within
biqtite schists found in Western Australia, and within pegmatites in
the New England region of New South Wales. Other gemstones
which occur in metamorphic rocks include ioUte (the gemquality
lilac-purple variety of cordierite), timite and kyanite. Jade is a
term given to tough compact aggregates of two minerals, i.e.
jadeite (a pyroxene) and nephrite (a term given to a group of
amphibole minerals, not a minerd).
Sedimentary deposit
By far the most valuable gemstone formed in sedimentary
envirqnments is precious opal. The largest deposits of this occur
throughout central Australia in the far western New South Wales;
muth-west Queensland; and central to northern South Australia in
what is known as the Great Artesian Basin. Most of these opal
deposits occur in fine-grained rocks of Cretaceous age.
Placer deposit
Because of their toughness and resistance to weatherin
erosion, gemstones arerelativeli abundant in some placer deposits.
Most of the sapphires from eastern Australia occur in placer t
deposits where they have become concentrated by alluvial
processes. Other important gemstones which commonly awur in
placer-type deposits include diamond, zircon, topaz, ruby, garnet,
agates and petrified wood.
Gemstone deposits Io Ethiopia
It is known that Ethiopia is endowed with suitable geol
m
environments to host all varities of gemstone, although not yet -., ,
been well investigated in detail. Nevertheless, miner s o z-
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