Opals are truly one-of-a-kind. Gemstone identification books will tell you that opals cannot be mistaken for any other gem. They glow and sparkle with a rainbow of colors, ethereal and enigmatic. To look into the depths of a precious opal is to perhaps see the way Impressionist painters saw, a world full of movement and color. Read on to learn all about this fabulous gem.
How Opals Are Formed
Opal forms when heavy rains carry silica gel into cracks in deep, dry sedimentary rock. When the water evaporates, the silica drys and forms an opal. This process explains why opals are commonly found in desert regions, such as Australia and Africa.
The flashing of rainbow colors seen in an opal is called “play-of-color.” This factor distinguishes precious opals from common opals. Opal does not form in a crystal structure, but is instead composed of many silica spheres that stack themselves into rows. These spheres have the same chemical composition of quartz.
As the silica spheres stack up into rows, light waves are able to bend around spheres and create a play-of-color. The size of the spheres and the way they are stacked determines how the light returns the color to you. The sphere size spectrum goes from .1 microns (violet) to .2 microns (red). The most valuable opal has an even distribution of play-of-color and has large patches of color. When an opal displays red, it is considered highly valuable.
Types of Opals
The Gemological Institute of America recognizes five main types of opals that they describe as follows:
White or light opal: Translucent to semi-translucent, with play-of-color against a white or light gray background color, called bodycolor.
Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
Fire opal: Transparent to translucent, with brown, yellow, orange, or red bodycolor. This material—which often doesn’t show play-of-color—is also known as “Mexican opal.”
Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.
Are Opals Unlucky?
Perhaps you have heard that opals are unlucky for those not born in October, or that opal engagement rings will result in a doomed marriage? You can credit writer Sir Walter Scott with the unlucky superstition surrounding this beautiful gem. In Anne of Geierstein (1829), he wrote about a tragic character who wore an opal in her hair that would ultimately be her undoing. This unfortunate tale is, however, an anomaly in the long history of opal lore. The Ancient Romans revered the opal, and regarded it as a symbol of love and hope. Since then, the gem was believed to hold mystical and curative powers throughout the centuries. The unlucky superstition is relatively recent, comparatively speaking, however it seems to persist, perhaps in part, due to the fragile nature of opals.
Opal is a soft stone, with a Mohs hardness of 5 to 6.5. It’s fragility also has to do with its water content, which is about 20%. When an opal’s conditions rapidly change, for example from wet to dry or from darkness to intense bright light, it is highly susceptible to a condition known as crazing. Internal cracks form and can cause the opal to split. Crazing can also be caused by vibration, which is why cutting opals is such a delicate process. This fragility does not make the opal unlucky, it simply means it needs to be treated with more care. Avoid wearing opal jewelry in harsh environments (i.e. don’t wear your opal rings when you do the dishes), and keep away from chemicals and dirt. Opals may be gently cleaned with warm, soapy water and a soft toothbrush.
Although they may require more care than other stones, opals are some of the most beautiful gems you can own. Market Square Jewelers hand selects our opals to ensure the highest quality before setting them into our jewelry. To shop our opal selection, click here.