Gemology 101: Introduction to Gemology
Article: Opal
The fragile opal first flashed its fiery colors at mortals millennia ago, igniting a love affair that has survived the fickleness of the human heart. Its faults, like softness and lack of stability, are overlooked by those entranced by its beauty.
Although comprised simply of silica and water, opal is one of the most complex gemstones known. Not only are opals unique among the members of the mineral family, no two opals are identical. The molecules in most gemstones form crystals, but the silica molecules in opals take the shape of tiny spheres. The silica spheres pack together tightly in precise layers. Although the spheres and layers are as close to each other as they can get, tiny spaces between them exist. Opal's uniqueness results from the minute differences in the sizes of the silica spheres and their spacing from one another.
The play of color, or fire, prized so highly in precious opal, is a special effect of light. All color comes from light, which may be absorbed or reflected back from solid substances like gemstones. The absorbed colors, "bouncing" around the silica spheres and the spaces between them, produce a brilliant play of color as light moves across an opal's surface.
Each type of color is named according to its appearance. Large square-shaped patches of color are called harlequin or mosaic. Flash fire describes large irregular patches of color. Flame fire is streaks of color, while pin fire refers to small pinpoints of color. Sheets of color that roll across the surface as the stone is moved are called rolling fire. Broad flash fire is sheets of color that cover the stone. Other descriptive names include zebra, Chinese writing, and straw. These latter varieties are rarely found.
Opal that doesn't have a play of color is called common opal or potch. Water, or jelly, opal is transparent, without a play of color, but has an iridescent glow that makes it desirable.
Gilson opal is a synthetic (manmade) which mimics all the chemical and physical properties of genuine opal. Slocum stones are imitation opals made from glass. Both glass and plastic imitations have been named Opal Essence. Opalite is made from plastic. Many of the imitations duplicate the play of colors so skillfully that they are difficult to distinguish from precious opal with the naked eye.
Although nearly all precious opal today comes from Australian mines, opal is found throughout the world. Within the United States, opal has been located in Arizona, Oregon, Idaho, and Nevada. Prior to the discovery of Australian opal, the Czerwenitza area of what was formerly known as Hungary was the major source of opal. Although the area has been renamed as political circumstances change, opal from the Czerwenitza mines is still known as Hungarian opal. Mexican fire opal contains iron, which gives it a distinctive red/orange color. It does not have the play of color that other opals have. Color descriptions in opal usually refer to the background, or base, color. That's the color seen when looking straight down at the opal. Although opal is found in a variety of colors, including yellow and blue, black is most highly prized base color. Additionally, opal may range from transparent to opaque. Their names indicate the opals' color and translucency. A black opal is opaque, with the play of color showing against a dark background. Black crystal opal is the same as black opal, except it is transparent or translucent. White opal displays its play of color against an opaque white background.
The name opal comes from upala, which is Sanskrit for stone. Opal has been valued since ancient times, its first use being to protect the wearer from diseases, particularly those of the eye. Opal was especially popular in the Middle Ages, when it was thought to make a person invisible. Negative superstitions later linked opal with death and bad luck. It didn't regain its popularity until Queen Victoria made it a Court favorite. Opal's fragility, as well as superstition, has contributed to its fluctuating popularity.
Opal is formed when ground water leaches silica from the soil. The groundwater accumulates in cracks deep within the earth where most
of water eventually evaporates, leaving the silica behind. After 15 million years or so, natural forces such as heat solidify the silica into opal. When mined, opal may contain up to 20 percent water, which continues to evaporate over the life of the opal. To counteract that evaporation, opal needs to be dipped in pure water frequently to keep it from dehydrating. If it gets too dried out, it will develop a network of fine surface lines (craze), and possibly crack. Because of its water content, extreme temperatures can cause opal to break. Opal is also soft and quickly scratched. Opals crack easily during polishing or from the pressure of being mounted in a setting.
Opal is very absorbent. It should never be cleaned with anything but pure water, because it will absorb anything, including other minerals, in the water. Putting an opal in an ultrasonic cleaner will destroy it. Hair sprays, perfumes, and cosmetics should be applied before putting on opal jewelry. Opals need to breathe and should not be stored in plastic bags. Wrapping them in soft cloth or leather both protects them and allows the fresh air they require.
Because of their fragility, opals are often sold as triplets or doublets. An opal triplet consists of a piece of precious opal sandwiched between a top layer of clear quartz or glass and a bottom layer of common opal. A doublet is usually precious opal underneath a quartz layer. The quartz primarily protects the soft opal from scratches. The layers are usually fused together with a colorless cement.
Opal-unique among gemstones. Fragile, but fascinating. Made of water, but filled with fire. Its brilliant beauty overcoming its faults, the fiery opal enchants with its dazzling display of color. Its special place in the human heart is secure.
By Sandra I. Smith, Writer
You assume all responsibility and risk for the use of the safety resources available on or through this web page. The International Gem Society LLC does not assume any liability for the materials, information and opinions provided on, or available through, this web page. No advice or information provided by this website shall create any warranty. Reliance on such advice, information or the content of this web page is solely at your own risk, including without limitation any safety guidelines, resources or precautions, or any other information related to safety that may be available on or through this web page. The International Gem Society LLC disclaims any liability for injury, death or damages resulting from the use thereof.
The All-In-One Jewelry Making Solution At Your Fingertips
When you join the Ganoksin community, you get the tools you need to take your work to the next level.
Trusted Jewelry Making Information & Techniques
Sign up to receive the latest articles, techniques, and inspirations with our free newsletter.