O is for Opal
My paternal grandmother was born in October and opal was her birthstone. I remember her telling me that an opal was a symbol of bad luck and I think she felt like she was destined to suffer bad luck because of it. I hope she did not truly believe that. She had many difficult events in her life, but I think many people have similar circumstances. I found an excellent article about gemstones that details some possible reasons for the belief opals are bad luck. You can read it here – it is quite interesting.
Opals are a form of hydrated silica comprised of up to 20% water. They are considered a mineraloid rather than a mineral because of their lack of crystal structures. Opals are formed when heavy rains carry silica down into deep layers of sedimentary rock. Upon evaporation, silica deposits are left behind.
There are two broad classifications of opals – precious and common. Precious opals display beautiful color flashes (known as play-of-color) while common opal, even though it comes in many different colors, does not have the vivid color flecks and flashes.
Rather than attempt to list and describe all the different types of opal (there are many) I will talk about how opal values are determined. I try to steer away from the hundreds of sources trying to sell opals to determine how to grade them. According to GIA, an opal’s quality is determined by color, pattern, and clarity.
There are five steps in evaluating an opal.
- Determine the type of opal.
- Evaluate the play-of-color
- Determine the transparency
- Evaluate the clarity
- Observe and evaluate the cut
I have a fondness for fire opal. You can see a lovely specimen at the Smithsonian by clicking here.
In addition to solid opals, there are also opals that are backed by other materials such as obsidian or glass. These opals are classified as doublets (a thin slice of opal cemented to a black backing) or triplets (a doublet with a clear domed top layer of quartz, glass or resin).
Opals are fragile and are not suited for prong setting. If set as a cabochon, it should be fully protected by a strong bezel and should not be exposed to any action that may crack the stone. The opal hardness ranges from 5 to 6.5 on the Mohs scale. A solid opal should only be cleaned with warm soapy water. Avoid high temperatures or extreme dry environments which may cause the opal to crack. Doublets and triplets should not be immersed but should only be wiped clean.
If you are Interested in seeing the many different types of opals, here is a good place to start.