There is here
no semicircular evanescence, as the specimen is rotated in azimuth. On
the contrary, the colored light transmitted perpendicularly through a
thin plate of opal undergoes no change when the gem is turned round in
its own plane. This appears to prove that the alternate states are not
related to one another as twin crystals. More probably the alternate
strata are of air, as in decomposed glass. The brilliancy of opals is
said to be readily affected by atmospheric conditions.
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