October Birthstone - Opal

Physical and Chemical Properties

Chemical composition

Silicon dioxide, SiO2, the same elements that make up quartz is also the constituent of opal. Opal is famous for its play of color.

The play of color is caused by diffraction of light by the ordered arrangement of closely packed microscopic silica spheres with water enriched spaces between them. The spheres are arranged in octahedrons (base to base pyramids). When the packing of the silica spheres becomes faulted, the striations produce colors that can be seen in reflected light. Play of color is sometimes erroneously called "fire." The term "fire opal" should be used to describe opals with a red body color; they may or may not show play of colors. Opal is neither very hard (5 1/2 to 6 1/2 on a scale of 10) nor very tough. It has a conchoidal to splintery fracture and is often very brittle.

Colors

The play of colors from the stone can occupy almost any wavelength in the visible spectrum but red and orange are more often preferred than yellow, blue or green. The body color of the opal may be white or tan or bluish. The finest opals have an even distribution of colors in fairly large splotches; pinpoints of light are less desirable. Gray or brown undertones detract from the stone. Some rare opals may show chatoyancy and produce a very fine eye and such stones are desirable even where play of color is insignificant. Opals are fundamentally colorless; various impurities add color to the stone, such as yellow and red from iron oxide and black from manganese oxide and organic carbon. Their milky appearance comes from gas-filled bubbles inside the tiny spherical network. Iridescent opals may also appear to be gray, green, blue, and orange due to the diffraction of light.

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