My gemstone of the day is opal, which also happens to be one of October’s birthstones.
The name “opal” is derived from the Sanskrit word for “stone.” It is a hydrous silicate mineral, similar to quartz, but with water molecules in its crystal structure. Opal occurs in a wide variety of colors and appearances. Opal is perhaps best known for the play of colors it exhibits, but there are varieties of opal that do not exhibit play of colors.
Opal has a hardness of about 5.5-6 on Moh’s hardness scale, but it is susceptible to fracturing. For this reason, I don’t recommend using opal in jewelry that undergoes a lot of wear and tear, such as rings and bracelets. Opals are ideal for earrings, pendants, and brooches.
Opals require special care. They must be stored moist; if an opal dries out completely, it is subject to crazing (cracking). Opal is often treated with oil, resin, or plastic to improve its durability. Opal is sensitive to heat, pressure, and chemicals. It should never been cleaned using an ultrasonic cleaner or steam. Opal is best cleaned using warm water with a little gentle soap.
Opals can be divided into three subgroups:
1. precious
2. yellow-red fire
3. common
Precious opal (Australian). Used with permission of gemsablaze.
Precious opals are the opals that most people are familiar with. They display play of color, and they are often white or black. The play of color is caused by microscopic crystobalite sphere suspended in a siliceous jelly. The spheres cause diffraction and interference patterns in light. Common enhancements of precious opal includes sugar treatment and smoke treatment. Both of these processes serve to enhance the play of colors. Precious opals are also commonly used in doublets or triplets (slices of gems adhered together to increase the size and value; the precious opal would be the top layer, with less-expensive gem material below).
Fire opals are often yellow, orange, red, or white. Not all fire opals display play of colors, but some do. Fire opals are mined in Mexico, Brazil, Guatemala, the US, and Australia.
Porcelain opal, a type of common opal.
Common opals do not display a play of color. They are often white, pink, or blue. They usually have a milky appearance.
More opal information:
Wikipedia
Webmineral
Mineral Gallery
micheledodge
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The precious opal really looks lit from within.
(p.s. I tagged you on my blog today–the challenge is to post 7 things about yourself!)