Volcanoes, galaxies, and fireworks. Those are just a few things that opal has been compared to by great writers. All of those are things of extraordinary beauty. Admirers have given opals poetic names like Pandora, Light of the World, and Empress.
In 75 AD, the Roman scholar Pliny marveled that this kaleidoscopic gem encompassed the red of ruby, the green of emerald, the yellow of topaz, the blue of sapphire, and the purple of amethyst. And it is because of this that opal has been regarded as the luckiest and most magical of all gems because it can show all colors.
The Romans are far from the only culture who held mystical beliefs about opal. Arabic legends say it falls from the heavens in flashes of lightning. Ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease.
In more recent times opal has become an October birthstone.
How Opal Formed
The product of seasonal rains of ancient time. They drenched the dry ground. Then the rain soaked deep into ancient underground rock beds, carrying
dissolved silica (a compound of silicon and oxygen) downward. During dry periods, much of the water evaporated, leaving now solid deposits of silica in the cracks and between the layers of underground rock. These silica deposits are what formed opal.
Known for its unique display of flashing rainbow colors called play-of-color. There are two classes: precious and common. Precious displays play-of-color, common does not.
Play-of-color occurs in precious opal because it’s made up of incredibly tiny spheres stacked—like layers of Ping-Pong balls in a box. When light waves travel between these spheres, the waves bend. As they bend, they break up into the colors of the rainbow. Play-of-color that you see is the result. The color you see varies with the sizes of the spheres. Spheres that are approximately 0.1 micron (one ten-millionth of a meter) in size produce violet. Spheres about 0.2 microns in size produce red. Sizes in between produce the rest of the colors.
Experts divide gem quality opals into five main types:
- White or light: Translucent to semi translucent, play-of-color against a white or light gray background color, called body color.
- Black: Translucent to opaque, with play-of-color against a black or other dark background.
- Fire: Transparent to translucent, with brown, yellow, orange, or red body color. This material is also known as “Mexican opal.” It often doesn’t show a play-of-color.
- Boulder: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
- Crystal or Water: Transparent to semitransparent, with a clear background. This type shows exceptional p
Contains up to 20% water trapped in its silica structure.
MINERAL: Hydrated Silica
COLOR: All colors
REFRACTIVE INDEX: 1.37-1.47
MOHS HARDNESS: 5 to 6.5
Pictures and information from GIA.
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