Opal is the stone people ask me about most, and also the one they understand least. The name gets used loosely, the types blur into one another, and a fair amount of myth has attached itself over the years. So here is the plain answer to the question I hear across the counter most weeks: what actually is an opal? After fifty years of cutting and setting them, this is how I explain it.
What opal is made of
Opal is hardened silica with water trapped inside it. Tiny spheres of silica settled and set over a very long stretch of time, and a finished opal still holds somewhere between about three and ten percent water by weight. That water is unusual for a gem. It is the reason opal sits a little softer than stones like sapphire, and the reason some of it asks for gentle handling. Strictly speaking opal is a mineraloid rather than a mineral, because it has no regular crystal structure. That last point matters more than it sounds, and I will come back to it.
Why an opal is not a crystal
People often call opal a crystal. In the geological sense it is not. A diamond or an amethyst grows in an ordered, repeating lattice. Opal does not. Its silica spheres pack together more like stacked oranges than like a rigid grid, and that disordered structure is exactly what gives the best opal its fire. Light works its way between the spheres and splits into color. Pretty rather than precise, which suits me fine.
Precious opal and common opal
There are two broad families. Precious opal shows that shifting rainbow flash we call play-of-color, and it is the one that ends up in fine jewelry. Common opal, sometimes called potch, has no play-of-color at all. It can still be a handsome solid color, and a little of it is used in jewelry, but it lacks the magic that makes collectors lose their heads. When someone says “opal” and means the gem, they almost always mean the precious kind.
What an opal looks like
The body of the stone, what we call its body tone, runs from black through gray to milky white and on to water-clear. Against a dark body the colors look deepest, which is why black opal sits at the top of the price tree. Against a pale body the same colors read softer and more pastel. The flashes themselves are most often blue and green, with orange and red being far rarer. I put the patterns side by side in my visual guide to what opal looks like.
Curious to see opal in the metal? Browse the handcrafted opal rings in my collection, or ask me about a custom piece built around a stone you love.
Where opal comes from
Australia has produced most of the world’s precious opal for more than a century and is still the benchmark. Ethiopia became a major source in the last fifteen years and brought bright stones at friendly prices. Mexico is known for warm fire opal, and there are smaller deposits in Brazil, the United States and a handful of other places. If you are curious how the stone actually forms underground, I cover that in how are opals made.
Is opal a birthstone?
Yes. Opal is the birthstone for October, which it shares with tourmaline, and it is also the traditional gift for a fourteenth wedding anniversary. That October tie is part of why it has stayed such a popular gift stone.
Frequently asked questions
Is opal a gemstone or a mineral? It is a gemstone, but technically a mineraloid rather than a true mineral, because it lacks the ordered crystal structure that defines a mineral. It is still very much a precious gem.
What is the difference between precious and common opal? Precious opal shows play-of-color, the rainbow flash. Common opal, or potch, does not, though it can be an attractive solid color in its own right.
Is opal expensive? It ranges enormously, from modest white opal to black opal among the priciest gems of all. Body tone is the biggest factor, with brightness and the amount of red close behind. I break it down in how much is opal worth.
That is opal in plain terms. If you would like to go further, my full guide to understanding opal covers the types, color and value in one place. Or browse the handmade opal rings in my collection and see the stone for yourself.
