Opals - Types
OPAL TECHNICAL DATA
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Species----------------------------Opal
Chemical
Formula--- --SiO2.nH2O
Birthstone
--------------------October
*Mohs
Scale Hardness- --#5.5-6.5
Durability---------------------------Fair
*Refractive
Index----------1.42-1.47
*Specific
Gravity------------------2.15
Significance
of These* Data (FAQ#5)
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OPAL ADDITIONAL INFORMATION
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Color
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The flashes of color exhibited by Opal are pure
spectral colors which change as the stone or light source are moved. The play
of color is created by the defraction of light from a perfectly ordered
arrangement of very tiny spheres of silica gel. The size and shape of the
flashes can very from pinpoint to broad flash.
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Inclusions
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The range of body color, from colorless through
white to cream to black is caused by the very fine foreign material in the
spaces between the spheres of the otherwise transparent silica gel.
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Sources
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Historically Australia has been and still is the
principal source of gem grade Opal. Other sources of Opal are Mexico, Nevada,
Idaho and Brazil.
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Treatment
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Some Australian Matrix Opal is treated with a
sugar solution followed by sulphuric acid to blacken the host material so as
to give it the appearance of Black Opal. Otherwise Opal is only modified by
the use of doublets and triplets.
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Care
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Opals should not be subjected to any sudden
temperature changes or extremes of heat or cold. Opal jewelry should be stored
individually so as not to be scratched by harder gemstones. Opals have a
tendency to lose their moisture and it is better to store very fine opals in a
humidor (a sealed plastic box containing a damp sponge).
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Uses
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Because Opal is brittle and not a hard stone it is
best used in a very well protected ladies' ring or for pendants or earrings.
Opal triplets with a quartz cap can be suitable for a well protected mens'
ring.
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Value
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The size of the gemstone and the quantity, size
and intensity of the flashes determines the value of the Opal.
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RETURN TO OPAL
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