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.
SUNSTONE Sunstone is a plagioclase feldspar, a variety labradorite. It often displays “aventurescence,” a type of schiller (glitter-like) effect that is the result of red copper inclusions. It may be called “aventurine feldspar” because of its aventurine-like appearance. Notably hailing from Norway, it can now be found in many locations, famously including Southeast Oregon. It is not treated.
PERIDOT Peridot is a very special stone – it has been found in meteorites! However, these specimens are too small to be used as gems. In 2005, comet dust containing peridot was brought back from the Stardust space probe. Peridot can be pronounced both with and without the “t” on the end. Peridot is the gem-quality version of the mineral olivine, which is much more abundant than peridot. It is an August birthstone. Peridot is not treated for color but may be oiled or waxed, or resin-filled, to improve appearance. This should be disclosed.
PIETERSITE Pietersite is an aggregate – a rock made up of fragments embedded in a matrix. It is composed mostly of Hawk’s Eye and Tiger’s Eye. It often has a dark gray or blue body color, and may have swirls of orange (crocidolite inclusions). It was first described in Namibia in 1962 and was named in honor of the father of the man who discovered it. It may display “chaotic chatoyancy,” or a sheen in spots where certain inclusions exist. It is not treated.
PREHNITE Prehnite has gained popularity in recent years due to the discovery of deposits in Africa, making it more accessible. Prehnite is named for Colonel Hendrik Von Prehn, who discovered it in the 1700’s in South Africa. He was a Dutch mineralogist. It often has epidote and/or copper inclusions. It is a natural stone and there are no known treatments.
PYRITE Pyrite, also known as “Fool’s Gold,” has long been mistaken for real gold. It is, in reality, quite different, although the two are often found together. Long ago Native Americans would polish pyrite and use it as mirrors. The name comes from the Greek word “pyr,” meaning “fire,” because it could be swiped against a rock to create a spark. It is not treated in any way.
QUARTZ Quartz is the second most abundant mineral in earth’s crust, and some well-known varieties include citrine, amethyst, and prasiolite. The word “quartz” comes from the Greek kruos, meaning “icy cold.” The Greeks believed quartz was a supercooled form of ice. It is used in many forms of technology due to its pyroelectric properties (it reacts to temperature change with positive or negative charges).
- CHALCEDONY Chalcedony is a microcrystalline variety of the mineral quartz. It is translucent to opaque and can be almost any color, but naturally it is often white-to-blue, bluish-gray, or brown-to-black. Many well-known stones such as agate, jasper, carnelian, and onyx, are actually types of chalcedony. Chalcedony has been used by people since at least 1800BC. It holds dye fairly well, and the bright colors of chalcedony, as well as the ever-popular aqua chalcedony, are the result of this treatment. It can be heated to create carnelian. When dyed green it is called green onyx.
- HERKIMER DIAMOND Herkimer diamonds are not diamonds at all, rather they are double terminated quartz. They are colorless to smoky with a wide range of inclusions. They come from Herkimer County, New York – these are the only ones that should be called Herkimer diamonds. Other double terminated quartz comes from Pakistan and may be referred to as “Perkimer diamonds” or “Pakinstan Herkimer.” It should be noted by the salesperson that these stones are not from Herkimer County. No matter where its origin, this stone is truly unique and is stunning in its natural form, without being cut.
- ROCK CRYSTAL Quartz is the second most abundant mineral in earth’s crust, and some varieties include citrine, amethyst, and prasiolite. The word “quartz” comes from the Greek kruos, meaning “icy cold.” The Greeks believed quartz was a supercooled form of ice. While quartz comes in virtually any color, colorless quartz is known as “rock crystal.”
- ROSE QUARTZ Rose quartz is a cloudy translucent pink variety of quartz. It is abundant and found in many locations all over the world. It’s highly cherished in the healing crystal community, notably for its effect on unconditional love. The hue of rose quartz is so treasured that it was the Pantone color of the year in 2016! It may be irradiated for color.
- RUTILATED/TOURMALINATED QUARTZ Rutilated quartz and tourmalinated (or tourmalated) quartz are similar and often confused. Rutilated quartz inclusions are rutile (titanium oxide), while tourmalinated quartz inclusions are tourmaline – just as the names suggest. It is important to distinguish correctly between the two. Rutiles are golden to brown to reddish golden, while tourmaline inclusions are typically black. While inclusions usually lessen the value of a gemstone, in these cases they are treasured.
- SMOKY QUARTZ As the name implies, smoky quartz is a variety of quartz. Quartz is a silicon dioxide crystal, and smoky quartz occurs when quartz is irradiated or heated, which results in free silicon. This can be natural or artificial.
- SOLAR QUARTZ Solar quartz is agatized quartz, cut from stalactites. It can be clear, white, or gray, and has mossy inclusions in a variety of earthy hues. Occasionally amethyst grows around the outside edges, creating a purple ring around the core. Solar quartz is often dyed bright colors.
- FRUIT “QUARTZ” Colorless materials with various colored “inclusions,” such as “cherry quartz,” “blueberry quartz,” “pineapple quartz,” “strawberry quartz,” etc. Almost all materials sold under these names are simply synthetic materials, commonly glass. Care should be taken when purchasing, if natural material is desired.
- STRAWBERRY QUARTZ Natural strawberry quartz gets its color from iron oxide inclusions. The inclusions often look vein-like. Most often however, strawberry quartz is a synthetic material, typically glass, and is disingenuously sold as natural.
- TANGERINE QUARTZ Real tangerine quartz is natural, unlike most “fruit quartz.” It is a red to orange variety of natural quartz, which gets its color from trace amounts of iron oxide. It is likely that all tangerine quartz comes from a single mine in Brazil.
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.