GOLD.
The
colour of gold depends upon the amount and type of
impurities it contains. Native gold is typically golden
yellow, but in order to nary its colour and increase its
hardness for use in juwellery, gold may be allowed to other
metals. Silver, Platinum, nickel, or zinc may be added to
give a pale or white gold; iron for a tinge of blue. Gold
purity is defined by the proportion of pure gold metal
present, and this is expressed as its carat (ct) value. The
purity of gold used in jewellery varies from 9carat (371/2
Per cent or more pure gold),
through 14, 18 and 22 carat, to 24 carat, which is pure
gold. In many countries, gold is "hallmarked" to
indicate its degree of purity.
Occurrence
Gold
is found in igneous rocks and in associated quartz veins,
often in small quantities invisible to the naked eye. It is
also concentrated in secondary "Placer" deposits
as nuggets or grains in river sands and gravels. Gold may
still be extracted from placer dopsits by traditional
planning method, but modern commercial mining involves large
earth-moving machinery and concentrated acids for processing
the ore. The main gold-bearing rocks occur in Africa,
California and Alaska (USA), Canada, the former USSR, South
America, and Australia.
REMARK
Gold
has been used for coins, decoration and jewellery for
thousand for thousands of years. It is attractive, easily
worked, and wears well.
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GOLD
NUGGET
Smooth,
waterworm surface |
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CRYSTALLIZED
GOLD NUGGET
This chinese
ear ornament has been worked in the shape of a
panda bear. The cracked surface is due to
dehydration of the stone |
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collected from
placer deposits by panning. |
GOLD
GRAINS
Typically
rounded and flattened grains.
Grains
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White Quartz |
GOLD
IN QUARTZ
MATRIX
Gold usually
occurs as fine grains, not groups of crystals
like these. |
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