Entries from October 2008 ↓

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

I’ve decided to just write a short post today, talking about the difference between gemstones that are labeled “natural” and those that are “not enhanced.” I’ve noticed a lot of confusion around these terms , and it’s easy to understand why.

A gemstone termed “natural” is a gemstone that grew via natural processes, usually in the Earth. (Organic gemstones such as pearls, coral, and amber are also natural, but not grown in the Earth.)

The term “not enhanced” means the the gemstone did not undergo any kind of treatment or enhancement to improve its appearance or durability.

A gemstone that was grown naturally, but then dyed or heated to change its color can still be termed “natural,” even though it was enhanced. For example, carnelian is a chalcedony that gets its orange color by undergoing a heating process. Although its color was changed by heating, it can still be called “natural.”

Materials that are not natural gemstones include synthetic gemstones and glass produced to imitate gemstones. Gemstones can be grown in a laboratory with an identical crystal structure and chemistry as a natural gemstone, but it should not be termed natural; these gemstones are called synthetic. (Common synthetic gemstones include hydrothermal quartz and synthetic emerald.) There are also many gemstone imitations that are made of glass that have gemstone-sounding names (for example, cherry quartz and goldstone).

The world of gemstones can indeed be a confusing world. As a consumer, it can be difficult to know what we’re buying with all of the confusing terminology used in the gem trade. Sadly, most gemstone suppliers don’t often seem to know what they’re selling.

That was my short post for today. Please post any questions you have in the comments area below. I love to talk about gemstones and the gem trade.

Coming up in the next week: Opals and Tourmaline!