Learn about opal and how it forms, in this video from NOVA: Treasures of the Earth: Gems. Opal, sometimes called the “queen of gems,” is primarily mined in Australia. Unlike most other gemstones that are made of crystal, opal is composed of tiny spheres of silica that are tightly packed together. The microscopic structure of opal scatters wavelengths of light to produce the different colors and fiery sparkle of the gemstone. How exactly opal forms is not well-understood, but it results from the interaction of water and silica. Opal forms under certain rare conditions as water percolates through the ground: the water dissolves silica from rock and forms a mineral mix that fills in cracks and gaps (and even fossils). When the mixture solidifies, it becomes opal.
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