Why Are Opals and Pearls Iridescent

A variety of minerals are prized for their exquisite beauty, rarity, and exceptional durability. These extraordinary materials are classified as gemstones. One such mineral, silica, with a chemical composition of Si02 (silicon dioxide), exhibits several crystal structures. Several gemstones are crystalline forms of silica, including amethyst, aquamarine, emerald, garnet, peridot, topaz, tourmaline, and zirconJ l 

Pearls are also considered to be gemstones composed mainly of calcium carbonate (CaCC 3). The luster associated with pearls is a consequence of the solid emulsion nature of pearls, with water again serving as the dispersed substance. 

Thermal analysis techniques reveal that water is bound in opal in more than one manner.t7 Most of the water is physically held in inclusions or microscopic pores within the opal, that is, in spaces between the microspheres. Water held in this manner can escape through complex systems of microscopic fissures or cracks, induced by temperatures even below 100°C. Some water is held within the opal via chemical bonding ( adsorption ) to the surfaces of the silica microspheres and is retained to temperatures approaching 1000°C.t7l Furthermore, since the microspheres themselves are composed of much smaller silica particles, water is additionally coated on the surfaces of these minute particles. The porous nature of opal and its thermal sensitivity require special care, for dehydration may result in cracking that greatly diminishes the value of this gemstone. 

KEY TERMS amorphous crystalline diffraction hydrate solid emulsion 

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