Gemologists, and actually most jewelry professionals, are a curious bunch. Staying inquisitive is the drive that keeps us looking for answers, no matter how long it takes.
And one can take the position that anything you think of actually exists somewhere – somehow.
So it is no wonder that the possibility of a star opal could cross our minds. Opals are forever fascinating owing to their original look. Every single opal is distinct in some sense. The way in which these natural gemstones crystalize is the reason for their individuality.
Maintaining Order
With most other gemstones, the crystal structure is organized in a certain orderly way. Many of our most popular transparent gem-minerals are constructed in a cubic (also called isometric) crystal system. Diamonds are in this category of course – but also are spinel, tsavorite, other garnets, and on and on. Within these crystal systems, the unit cells take the shape of a cube.
But, when it comes to opal, it’s a whole different story. This natural silica-rich gemstone resulted from hot mineral-rich silica oozing into rock cavities – over millions of years ago. Tiny spheres of silica bond together and eventually create opal. To give you a visual perspective, the internal structure of opal appears like the countless miniature soap bubbles stacked on top of each other in a lavish bubble bath.
Light Treatment
Their unique structure allows them to radiate a rainbow of hues in an otherworldly random pattern across the face of the opal. The opal stone is breaking up light into its complete spectral hues. To best exploit this spectacular display, opals are cut en cabochon with smooth rounded top surfaces and flat undersides. The colorful, shimmery patterns resulting from this type of polishing thrill gemstone fans.
Once Upon a Star
But could there be such a thing as a star opal? Since there’s so much randomness inside the opal, could a star phenomenon ever occur as well?
To uncover that answer, we have to understand how star stones happen. With most transparent to translucent gemstones, when natural silk like inclusions are present in the crystal, a sharp-eyed stone polisher will cut the stone into a cabochon with a high dome rather than faceting it. By doing so, the silk-like inclusions will begin to display on the surface of the stone as separate long rays – ideally going all the way down the sides of the stone.
In general, cutters aim for 6-rayed stars – the straighter the rays the better and the longer they are, the more desirable. Some stones will produce just 4 rayed stars. It’s less common, but on occasion, some 12 rayed star cabochons have popped up. These occur in black sapphire usually.
Opal’s Different In Many Ways
With opal however, because the crystal structure is amorphous instead of the orderly cubic structure, something else is going on below the surface.
According to GIA labs, which saw one such natural star opal, there’s a clear explanation found when you examine this anomaly under high powered magnification.
According to GIA’s research lab, “Microscopic examination revealed large parallel planes with play-of-color intersecting to form a hexagonal pattern. This is responsible for producing the six-rayed star. This unusual stone serves as a reminder that, unlike asterism in other gem materials, the star in opal is caused by diffraction of light from faults or imperfections in the packing arrangement of silica spheres.”
As Odd As We Are
While there may not be an abundance of these odd stones, they serve as a fascinating reminder that we haven’t seen it all yet – and that gemstones found in nature can be as individual as our human population on earth too.
