What is the Difference Between a Precious and a Semi-Precious Stone?

Although opals are mined around the world, Australia is synonymous with opal because the country produces about 95% of the world’s precious, gem-quality stones. Today, Australia’s major opal mining operations are concentrated in the eastern half of the country, in the Great Australian Basin located in Queensland and New South Wales.

Australia’s arid “outback” provided the perfect conditions under which opals formed. There, over millions of years, seasonal rains drenched the dry ground and soaked deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) into cracks and voids. As the water evaporated, it left behind silica deposits. This cycle repeated over millennia; however, only when conditions were ideal did grid-like layers of silica spheres, under gravitational pressure and a rising and falling water table, begin to form precious opal.

Rough Opal in Matrix

Unlike common opal, precious opal displays play-of-color. This occurs because light waves traveling between the microscopic stacked silica spheres are bent to refract different colors of the rainbow. Color differences among precious opals are dependant on the conditions under which each opal was formed. Gem-quality opals are assessed according to their clarity (transparent, translucent, opaque), their body color (the darkness of the opal regardless of the brightness or refracted colors), and the pattern or arrangement of the play-of-color. Among the common terms for play-of-color patterns are pinpoint, mosaic, flame, and peacock.

llyn strong fine art jewelry offers three types of gem-quality, Australian opals: boulder, crystal, and black.

Mined primarily in Queensland, boulder opal combines precious opal with the host rock in which it formed. The opal formations—thin seams or patches—are surrounded by and attached to the host rock, which in Australia is usually ironstone. The opal material in these seams is too small to isolate and fashion into a gem composed of solid opal, so the cutter creates a gem that includes both precious opal and the host rock. The gem can be cut in one of two ways. The first is across the seam to present a full face of precious opal with ironstone as the backing. Alternatively, the gem might be cut to display the layers of opal seams and patches alternating with the layers of host rock.

In contrast, crystal opal and black opal gemstones are composed of solid opal. The term crystal refers to any kind of opal—black, semi-black, white—that has a translucent, semi-translucent, or transparent body. This quality is also referred to as the diaphaneity of a stone. If you are able to see light through an opal, you probably are looking at a crystal opal. Crystal opals can display any color of the spectrum, and often display greater clarity and vibrancy of color than opaque stones.

For both connoisseurs of fine jewelry and investment collectors, the most valuable type of opal is the black opal. Noted for its high quality and rarity, black opal is mined only at Lightening Ridge, in Queensland. Black opal has the darkest base body color, ranging from dark grey to jet black, against which the spectral colors present in the opal appear extremely vibrant. Yellow, blue, and green are the most common displays of color in black opals; violet, fiery red, and orange are the most sought after of opal colors.

Precious opal jewelry requires some special attention. With a Mohs Hardness of 5 to 6.5, precious opal is vulnerable to scratching, so it’s more suitable for earrings, pendants, and brooches. If used in a ring, the setting should protect the opal. Opal contains up to 20% water trapped in its silica structure. If an opal loses moisture, crazing—a fine network of cracks—can occur. This loss can be caused by heat or excessive dryness, or exposure to bright light or extreme temperature changes, so opal jewelry should not be exposed to these conditions. Never clean opal jewelry in a mechanical cleaning system. The safest at-home method is with warm water, mild soap, and a cloth or very soft brush (not a toothbrush!).

llyn has designed and handcrafted a variety of rings, bracelets, pendants, studs, and earring jackets in 18k gold that feature vibrant, Australian gem-quality opals, all of which are available for purchase in our store on Main Street in downtown Greenville or on-line. And you’re invited to make an appointment with llyn to peruse our collection of loose stones and speak with her about designing a unique, custom piece artistically crafted to your style. If you don’t see a stone that “speaks” to you, Sydney Strong, our Doc of Rocks, can source a natural Australian precious opal perfect for your forever jewelry.

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