Gemstones from solutions

Photonic crystals have only been studied in the laboratory for two decades, but naturally occurring examples exist, with the best known being the gemstone opal. Opals consist of tiny spheres of silica arranged in a face centred cubic structure. These are thought to have formed from colloidal silica solutions, and the colour depends on the size of the spheres. 

Have you ever watched crystals slowly form from a water solution Sometimes water molecules adhere to the ions as the solid forms. These water molecules become part of the crystal and are called water of hydration. Solids in which water molecules are trapped are called hydrates. A hydrate is a compound that has a specific number of water molecules bound to its atoms. Figure 11-12 shows examples of a common gemstone called opal, which is composed of silicon dioxide (Si02). The unusual coloring is the result of the presence of water in the mineral. 

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