Gemology 101: Opals

Opals are truly a one-of-a-kind gemstone. With their ever-changing colors and unique, neon hues, they stand apart from all other precious stones and minerals. While diamonds or rubies may be known as rarer or more expensive, for true opal fans, nothing can come close. Gemology 101: Opals

Whether or not you’ve admired opals, in jewelry settings or on their own, in the past, if you’re interested in learning more about these gemstones, here’s everything you need to know.

What are Opals?

Opals are formed when seasonal rains seeps deep into dry ground, carrying dissolved silica. When the seasonal rain departs and the dry season kicks in, the water evaporates, leaving behind hydrated silica deposits that are now opal.

Opals come in two varieties, precious and common. What’s the difference? Precious opal features what’s known as “play-of-color,” or an optical effect that appears as flashes of colored light. Precious opal is what you’ll find used in jewelry.

Beyond precious and common opal, opals can be further divided according to their colors. White or light opal is almost translucent, with differing inner colors set against a white or light gray primary hue. Black opal is the opposite, with a black or otherwise dark primary hue. Fire opal, or Mexican opal, features a brown, yellow, orange or red primary hue; it’s rare to find fire opal that’s precious rather than common. Crystal opal, or water opal, is completely clear and shows a wide array of colors.

Boulder opal isn’t categorized by its colors, but rather by the fact that it’s surrounded by some of the opal’s previous rock, or matrix.

Opals in History

As you could likely imagine, opals, with their fiery colors and ever-changing appearances, were something of legends and myths in ancient times. Many cultures have held opals in high regard.

The word “opal” stems from the Roman “opalus,” which means “precious stone,” and the Romans considered opals to be some of the finest gemstones available. Arabic cultures said that opals were formed from lighting. The Greeks said opals held powers that could ward off diseases. Other European cultures connected opals to positive values such as purity and honesty

Other interesting beliefs surrounding the gemstone? That it’s unlucky for anyone not born in the month of October (opal is an October birthstone) to wear it and that wearing opal can preserve and protect blonde hair.

What to Look for in Opal

So what makes a singular opal stand apart as high-quality? While a gemologist might grade gemstones like diamonds by the four C’s — cut, clarity, carat and color — a gemologist grades an opal by its color, clarity and pattern. To evaluate an opal, a gemologist will first determine the type of opal it is, according to its color; then consider its play of color and how intense and prevalent it is; grade the opal’s transparency and clarity; and, lastly, consider the opal’s cut.

The most valuable and prized opal color is black. Black opal is the rarest variety and the play of color is more easily visible against the black background.

When grading play of color, the two most important factors a gemologist will consider are strength and range; the colors should play very brightly and include the entire spectrum of colors (though this is very rare; many opals will just display three or four colors total).

In terms of the play of color’s individual colors and the preference for some hues over others, the most valuable inner color in an opal is red, followed by orange and then green. The most valuable and high-quality opals will display these colors from every angle.

The play of color pattern is also considered. Potential patterns include pinafore or pinpoint, which features small, close patches of colors; harlequin or mosaic, which includes more broad and angular patches of colors; flame, which displays flame-like bands or streaks across the stone; and peacock, which is mostly blue or green, regardless of patch distribution. Spots in the pattern where no colors are visible are called dead spots or extinction. Dead spots can, as you might expect, bring down the value of an opal.

When looking at an opal’s clarity, a gemologist will, as when looking at most gemstones, look for a lack of inclusions. Since an opal can range from transparent to opaque, and some types of opals are naturally opaque or transparent (such as crystal opal, which is always transparent, compared to black opal, which is nearly always opaque), it’s not enough to merely say that transparency is preferred. Other factors such as the stone’s primary color must also be considered. However, a cloudy or marred background is never preferred in any case.

When grading an opal, a gemologist will also look for imperfections such as fractures, blemishes, pits or cracks.

Grading opal cut is somewhat difficult, as opals aren’t always cut in traditional shapes that you might see used when cutting diamonds or rubies. Instead, opals are cut in whatever way makes the most sense for the colors, pattern and clarity. A freeform shape may be used by a jewelry designer, as that is simply what makes the stone look best. Still, symmetry is preferred.

While carat size doesn’t necessarily reflect quality when it comes to opals, though it does impact price, it’s worth noting that opals are rather lightweight, so you can have a large stone that doesn’t weigh that much in terms of carats. This makes it quite easy for you to wear larger opals, if you prefer a big stone, but don’t want to be weighed down by a heavy piece of jewelry.

Famous Opals

Famous opals throughout history have included the Olympic Australis, the world’s largest and most valuable gem opal, discovered in South Australia and measuring 11 inches long, 4.75 inches thick and 4.5 inches wide. The opal has never been cut, weighs 7,000 carats and is valued at more than $2.5 million (in 1997 money).

Others include the Queen’s Opal, or the Andamooka Opal, which belongs to Queen Elizabeth II; the world’s first named opal, the Burning of Troy, which was presented to Josephine by Napoleon, and which is now lost; and the Halley’s Comet Opal, the world’s largest uncut black opal, measuring nearly 2,000 carats. In 2006, it was on sale for $1.2 million.


Need More?

Need to learn more about your favorite gemstones? Check out all of the Gemology 101 blog posts on the Yamron blog and be sure to subscribe to the Yamron newsletter for all of the latest and greatest in gemology and jewelry trends. Stop by our newly renovated boutique to check out our collection of jewelry. 

Sources and Further Reading