This week I bought an opal on Ebay- I like to troll the vintage category because sometimes I can score a really nice piece for very little money. It was a little hard to identify, though- and it got me thinking that it'd be fun to write a post about all the different types of opal out there right now.
I'm no scientist, but opal is opal because of it's physical properties- hydrous silicon dioxide (or simply- silica). It is not a crystalline form- it's amorphous. It is found in 2 basic location types- in sedimentary deposits (as in Australia) or volcanic deposits (as in Mexico). It has anywhere from 5% to 10% of water in its composition. In precious opal, the spheres of silica are arranged in an orderly fashion, and when light hits them, a prism effect occurs and produces the gorgeous play of color opal is known for.
Above are a couple of cabs of blue Peruvian opal. This opal does not have any play of color that opal is famed for, but because of it's lovely blue color, it is still a desirable stone. It's actually the national stone of Peru, and unfortunately lots of imitation/dyed stones have reached the marketplace to take their share of the pie. The prices for Peruvian blue opal have really taken off. Here's an article about how to spot 'fake' Peruvian blue opal:
The same can be said for Peruvian pink opal- very attractive color- but no play of color. Common opal comes in many more colors- yellow, green, purple and more....
This fire opal from Mexico has a distinctive orange color and although rarely has play of color, is prized for it's color (which can range from yellow to red) and it's translucence. Here's a nice article about Mexican fire opal:
The opal cabochons above have a white color base and a play of color. These cabs are from Cooper Pedy in Australia. Like other stones, opals are often named for the area in which they are mined. Here is a map and description of the major opal fields in Australia:
This is another example of Cooper Pedy. Sometimes, the base is white, sometimes it's translucent, and sometimes a combination of both. This stone has a base of softer sandstone. Much of the opal found in Australia has a similar base. In ancient times, Australia was underwater. Many shells and sea creatures have been replaced over time with fossilized opal. Mining for opal in Australia is hot, dusty, and hard work. Deep dark caves have to be dug in order to find a small amount of precious stone. Here is a cool video that will give you a snippet of what it's like to mine for opal:
The example above on the left is an example of crystal opal- where the base color is transparent. This is my favorite type of opal- it looks like clear water with fire inside! The stone on the right is semi-translucent and has amazing fire.
Sometimes, the base color of an opal is dark- or black. These are examples of black opal. These are highly-prized because the dark color really shows off the stones' play of color.
In addition, like the above stones, sometimes a thin slice of precious opal is glued to a dark backer stone in order to look like a more solid stone. It also brings out the fire in the thin slice. These are known as doublets or triplets. Here's a description of them:
These Australian stones are referred to as boulder opals (or Koroit or Yowah). The precious opal in these stones is intermixed within the base ironstone rock, and is not a solid piece of stone. These have their own picturesque beauty, however, and are also highly sought-after.
The stone on the right is a piece of Australian Andamooka matrix opal. This is so-named for the area in Australia it is found. It is a light colored limestone, that's been treated and turned dark, in order to bring out the exciting pin-point fire in the stone. The Fairy opal on the left is a similar matrix opal, but it's sandstone base is quite soft, so after it's been darkened, it's typically hardened as well.
Honduras is another location for fine matrix opal. These stones are 'sugared' in a heated solution, and then 'carbonized' to turn the sugar black. It's a lengthy process. The Fairy opal is not as prized as the Andamooka or Honduras matrix opal. Here's a great link to how matrix opal forms and is treated:
These are carvings made from Australian opal.
A relatively new source of firey opals is the Welo area of Ethiopia. The stones have amazing fire and have been the latest 'big thing' for opal collectors. Many of the stones are faceted. Some Ethiopian opal is dark/chocolate brown with fire.
Here's an Ethiopian opal I cut myself. The Ethiopian opal differs from Australian opal a bit. Whereas the Australian opals do not change much in water, these Ethiopian opals may become opaque when wet. When they've dried out, they return to their original state. They are more able to absorb and evaporate water from their pores.
All types of opal are a bit soft- like glass. Okay if dropped on a carpeted floor, but probably not on a hard surface. Be careful! Also, the worst thing you can do for opals is keep them in a safety deposit box- too dry! Wear them instead! Keep oils and dirt off of them- that could be absorbed and affect the play of color. No need to keep them in water- just wear them regularly and enjoy them!
Because opal is so sought-after, there's a market for imitations that look convincingly real. Above are opal beads that are man-made, created, or 'lab' opal. They have a variety of base colors, and have nice play of color. Sometimes it's hard to tell real from fake, but lab opal often has too regular a pattern to be real, and has a directionality to the fire. Their make-up is chemically just like their stone counterparts. Here's a bit more information about them:
These 'opal' beads are commonly referred to as 'opalite'. They have no play of color, are very inexpensive, and have the milky quality of crystal or transclucent opal. Here's a nice short article about them:
That's a good primer on opals! The Queen of Gems!