OPAL Opal is one of the most mysterious and treasured gems through all of history. In ancient Rome, it symbolized love and hope. The ancient Greeks believed opal would bestow upon its owners the gift of prophecy and would guard them from disease. Europeans have considered opal a symbol of hope, purity, and truth. The flashing color effect within some opal is called “play-of-color.” Opals with this optical phenomena are called “precious opals.” This effect is the result of the way silica spheres are arranged within the opal structure. “Common opal” is composed of amorphous hydrated silica, which makes it opal, but they never display play-of-color. Since opal lacks a crystal structure, it is technically a mineraloid, not a mineral. There are many types of opal. Some may be dyed. It is an October birthstone.
- BOULDER OPAL Boulder opal is a specimen that contains seams or patches of precious opal within its host rock. Australia is the most famous source of boulder opal (especially the Koroit Opal Field in Queensland), and it is also found in Mexico and Honduras, among other places.
- CANTERA OPAL Opal from Mexico that falls under the category of boulder opal, is it is opal still contained within a pink rhyolite host rock. Normally shaped like an egg or dome, the opal peers through the host rock from the center of the rock.
- DENDRITIC OPAL “Dendritic” refers to the fern-like inclusions in this variety of common opal. They are manganese, iron, or another metallic oxide and they create unique branch-like patterns within the stone. These gems may also be called “moss opal.” In the healing crystal community, it is known as “merlinite.” Most come from Australia.
- FIRE OPAL Neither precious opal nor common opal, fire opal is a variety of opal with a bright orange or red color. It is rare, but some may display play-of-color and therefore be called “precious fire opal.”
- PINK or BLUE OPAL Pink or blue opal is “common opal” – they do not display play-of-color. Pink opal has the scientific name “silicified palygorskite”, and may be a combination of opal, polygorskite, and chalcedony. Peruvian pink or blue opals come from Peru, just like the name implies. They were long ago found in the Andes mountains and considered a gift from Pachamama, the Inca goddess of fertility, “Mother Earth.” Pink or blue opal may also come from Australia, or Mexico. A type of blue opal is also found in Oregon, USA. In many cases, common opal is dyed for color, as the natural color can be quite washed out. Blue opal often has an iridescent property. Dyed chalcedony sold as Peruvian blue or pink opal is common and care should be taken to buy only from a reputable source. Pink opal is harder (6 on MOHS) than blue opal (5.5 on MOHS), and is the hardest opal in the world.
- OPALIZED WOOD/WOOD OPAL Opalized wood is a kind of petrified (“turned to stone”) wood in which the wood content has been replaced by opal. The carbon content of the wood is replaced by hydrated silica. Much of it is found in the United States, Australia, and Indonesia.
- WELO OPAL Welo opals come from a single location in Ethiopia (the Wollo Province) which was only recently discovered in 2008. They have since uncovered a second deposit, also in the Wollo Province. They are highly sought after for their excellent play-of-color. Although these Ethiopian deposits are “new,” opal relics located in a cave in Kenya were thought to come from Ethiopia as long ago as 4000 BC. Any color of Welo opal besides white and pale yellow are the result of dying. Nearly all of the black Welo opal on the market has been treated – either smoked or sugared. Natural black opal is very fragile, so treating other colors of opal is the best way to achieve opal with black body color.
*SPECIAL CARE FOR WELO OPALS: Welo opals are “hydrophane,” meaning they can absorb water because of their porosity. If this happens, it can cause cracking as well as cause the play-of-color to disappear. The play-of-color will often return as the stone dries, which can take days to weeks. Since they are so absorbent, they can also absorb things like lotions, perfumes, and sweat. Please do not allow your opals to make contact with water, lotion, perfume, or anything that they may absorb. If they need to be cleaned, wipe them with a microfiber cloth.
Much of the information in this post came from my personal knowledge. I also used www.Mindat.org, www.Gemdat.org, www.Geology.com, www.Minerals.net, Wikipedia, and of course www.GIA.edu. I recommend all of these sources as reliable and trustworthy.