The
tract of the Burma ruby mines forms the most important area for rubies
and spinel in the world, and they have been found in the extensive
bands or lenticles of crystalline limestone or marble. These mines were
known to the Europeans from the fifteenth century. Due to the
denudation of enormous quantities of rock, the gem-stones have been
concentrated in pockets in the debris in mud filled cavities, and
caverns such as are well known in the limestone districts. The
principal workings have so far been in alluvial deposits in the valley
bottoms. The gem bearing gravel is known in Mogok as byon. The byon or
ruby strata is either alluvial or a rotten rock between the limestones
and the tongues of granite intrusions which form little zones of
pneumatolysis. The granite-limestone contact is always held favourable
for the byon. The associated minerals in the byon are ruby, sapphire, spinel, chrysoberyl, tourmaline, garnet, zircon, beryl, quartz, scapolite, apatite and danburite.
The
ruby mines are also famous for sapphires which are found like the ruby
in the alluvial deposits. The Mogok valley has produced a few
magnificent sapphires, though specially reputed for its rubies. It is
not found in the limestone as the ruby but is found in felspathic and
weathered igneous rocks and corundum syenite*.
Gem Mining
Mogok area.—Sapphire
is found in the ruby mines from near Kathe, at Kyaungdwin near Kathe,
from Ingaung near Gwebin and Kabaing. The majority of the finest
sapphires were obtained from this area. Bernardmyo and Chaunggyi also
produced some good sapphires. Sapphires also occur in hill-side or
valley deposits. Sapphires attain much larger sizes than the rubies.
Stones of 293 and 630 carats weight were found at Kathe in 1930 and
1932. Mogok produced » stone of 514 carats, and at Gwebin a stone of
1,000 carats in 1929. They are
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