Opal is considered an October birthstone. Some people think it’s unlucky for anyone born in another month to wear an opal. That superstition comes from a novel written in the 1800s (Anne of Geierstein) by Sir Walter Scott), and not from any ancient belief or experience. In fact, throughout most of history, opal has been regarded as the luckiest and most magical of all gems because it can show all colors.
Because opal has the colors of other gems, the Romans thought it was the most precious and powerful of all. The Bedouins believed that opals contained lightning and fell from the sky during thunderstorms. Writers have compared opals to volcanoes, galaxies, and fireworks.
Opal is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The showers soaked deep into ancient underground rock, carrying dissolved silica downward. During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
Opal is known for its unique display of flashing rainbow colors called play-of-color. There are two broad classes of opal: precious and common. Precious opal displays play-of-color, common opal does not.
Play-of-color occurs in precious opal because it’s made up of sub-microscopic spheres stacked in a grid-like pattern—like layers of Ping-Pong balls in a box. As the light-waves travel between the spheres, the waves diffract breaking into the colors of the rainbow, called spectral colors.
- MINERAL: Hydrated Silica
- CHEMISTRY: SiO2nH2O
- COLOR: All colors
- REFRACTIVE INDEX: 37-1.47
- BIREFRINGENCE: None
- SPECIFIC GRAVITY: 15 (+0.08, -0.90)
- MOHS HARDNESS: 5 to 6.5
Individual opals can vary widely in appearance and quality. As diverse as snowflakes or fingerprints, each gem can differ noticeably.
What to look for:
Color Color refers to background color and play-of-color. Pattern is arrangement of play-of-color.
Clarity With an opal, clarity is its degree of transparency and freedom from inclusions.
Cut The cutter considers an opal’s color, pattern, and clarity when planning the finished gem. Exceptional opals might not be cut to standard sizes and shapes. Domed surfaces give the best play-of-color.
Carat Common sizes for many of the opal cabochons set in jewelry are 6×4, 7×5, and 8×6 mm. Opal however comes in a wide range of sizes.
Caring for your opal:
Durability– Opal is a gem suitable for jewelry but requires care when wearing to not scratch or break the stone.
Care and Cleaning– Warm soapy water is always safe. Don’t expose an opal to extreme light, heat, or dryness even though most opals are cured to make sure it won’t crack.
Treatment– Opals can be treated by impregnation with oil, wax, or plastic, and by surface modifications.
Opal Gemstone | Opal Stone – GIA. (n.d.). Retrieved July 27, 2016, from http://www.gia.edu/opal