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Roman gem historian extraordinaire, Pliny, wrote in 79 AD that opal combined the beauty of all the other gems: “There is in them a softer fire than the ruby, there is the brilliant purple of the amethyst, and the sea green of the emerald—all shifting together in incredible union. Some by their splendor rival the colors of the painters, others the flame of burning sulphur or of fire quickened by oil.”

Opal’s play of color is unique in the gem world: color moving in shifting iridescent planes within the stone.

Opal forms in sedimentary rocks when silica-rich water slowly seeps into the host rock, filling seams and crack and hollows. In Australia, this happened about 60 million years ago in the Cretaceous Period. If the water then hits a non porous layer of rock that stops its progress and sits, perhaps for thousands of years deep within the earth, the silica will settle and eventually form a solid gel, trapping the remaining water within its structure. Volcanic opal forms more quickly, with silica rich water filling cavities under heat and pressure.

The play of color in precious opal is caused by stacked grids of spheres of silica within the stone. When an opal has spheres that are the same size, the silica spheres and the space between them form a three dimensional lattice that acts as a diffraction grating, breaking light into spectral colors. The colors displayed in each stone depend on the size of the spheres: small spheres can only create blue. The largest can show red as well as all the other colors, which have shorter wavelengths.

The same kind of diffraction in two dimensions creates the rainbow iridescence on the surface of a CD: the fine engraved lines of the recording break up the light into interference colors.

The value of the different opal varieties is based on how dramatically they display their unique patterns of color.

Opal’s play of color is more visible when the body color of the opal is dark, throwing the color into contrast. Black opal, which is more rare, is the most valued type of opal. Color is also more visible when the opal is clear, or “crystal,” rather than cloudy.

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