What Gives Opal its Array of Colors?

With all we know about gems and the millions upon millions of years it takes to create them, one gem still escapes understanding.  The opal.  Exactly how an opal is formed cannot be fully explained by science, and that’s one reason why they fascinate us so.

The first thing you notice about an opal are the colors.  Not just some colors.  Absolutely every color imaginable.  As you move and turn an opal, iridescent colors shift from red, green, blue, yellow, purple, and any other color you can imagine.  Jewelers call this an opal’s “play-of-color.”  This is caused by light diffraction, like how a prism will split white light into all the colors of the rainbow.  

Photo credit: gia.edu

Only within the last half-century have scientists figured out what causes this.  Through the years, silicates in the soil dissolve during heavy rains.  This solution seeps into cavities and deposits itself where it eventually forms tiny silica spheres.  It’s these spheres that cause the diffraction of light and produce the many colors.  Opals can be found on most continents, but Australia holds the title as “home of the opal.”  Seasonal rains that fall upon the dry outback creates the perfect place for the formation of opals.

As with all gems, the formation of an opal takes requires tens of thousands of years.  As the rains soak deep into ancient underground rock and deposits the silica, water eventually evaporates and leaves behind the solid deposits we call opal.  

Photo credit: gia.edu

Opals can be a single color, two or three, or show off all the colors of the rainbow.  The best opals are the ones with a play-of-color that displays a full spectrum of bright colors.  Ideally, the colors are uniformly distributed across the full face of the gem with no dead spots, and you can see the play-of-colors from any angle.  It’s very rare to find an opal with all those qualities in place, but when you do it is known as a “precious opal,” and is particularly valuable.

Black opal 18K gold earrings from our Estate collection

Opals will always start a conversation, and you’ll never see two that are alike.  They have been admired by people all over the world for centuries, and at one point in time rivaled diamonds in popularity.  October’s birthstone is a true miracle of nature, and many aspects remain a mystery to many geologists.

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