The process can be described by Bragg's law of diffraction.įor gemstone use, most opal is cut and polished to form a cabochon. The colors that are observed are determined by the spacing between the planes and the orientation of planes with respect to the incident light. Where the distance between the regularly packed planes of spheres is around half the wavelength of a component of visible light, the light of that wavelength may be subject to diffraction from the grating created by the stacked planes. The regularity of the sizes and the packing of these spheres determines the quality of precious opal. Sanders in the mid-1960s that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal. At microscopic scales, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packedlattice. Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. Hot salt water, bases, methanol, humic acid, hydrofluoric acid Mexican opal may read as low as 1.37, but typically reads 1.42–1.43 īlack or white body color: inert to white to moderate light blue, green, or yellow in long and short wave, may also phosphoresce, common opal: inert to strong green or yellowish green in long and short wave, may phosphoresce fire opal: inert to moderate greenish brown in long and short wave, may phosphoresce Single refractive, often anomalous double refractive due to strain Colorless, white, yellow, red, orange, green, brown, black, blue, pink
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