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The Virgin Valley opal fields of northern Nevada produce a wide variety of precious black, crystal, white, fire, and lemon opal. The black fire opal is the official gemstone of Nevada. Most of the precious opal is partial wood replacement.
Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At microscopic scales, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. It was shown that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the micro-structure of the opal.
The regularity of the sizes and the packing of these spheres determines the quality of precious opal...Read More