Opal has been a popular gem for many centuries and has a very interesting structure. Opal is considered a mineraloid because this structure is not truly crystalline. The chemistry of Opal is primarily SiO2 and varying amounts of water. The amount of water varies from 5 -10% and greater.
This is an opal doublet which is
athin layer of opal backed by onyx.
Opal is the birthstone for October.
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