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Opal Specimens
Precious opal shows a variable interplay of internal colors and does have an internal structure. At the
micro scale precious opal is composed of silica spheres some 150 to 300nm in diameter in a hexagonal or cubic closed-packed
lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing
through the microstructure of opal. It is the regularity of the sizes of the spheres, and of the packing of these spheres
that determines the quality of precious opal. Where the distance between the regularly packed planes of spheres is approximately
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 spacing between the planes and the orientation of planes with respect to the incident light
determines the colors observed. In the Middle Ages, opal was considered a stone that could provide great luck because
it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.
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"Ironstone" matrix chunk with a layer of opal running across part of the surface from Australia. There is a little
color flash in the very center of the stone, however it's very slight and you have to be holding the specimen on the right
angle to see it. I can attest to this still being a very interesting piece though. It's one of the most picked
up and examined pieces in my shop. Approx. size: 99x60x41 Weight: 225 grams
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