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Once colloids get as large as about 240 x 10-6 mm, red color is seen (Carr et al., 1979). Not all opals have the famous play of colors, however. Common opal has a wax-like luster & is often milky whitish with no visible color play at all. Opal is moderately hard (H = 5 to 6), has a white streak, and has conchoidal fracture.
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Opalescence or play of color is an optical phenomenon associated with the mineraloid gemstone opal, [1] a hydrated silicon dioxide. [2] This effect appears as a milky, translucent glow that changes with the angle of light, often creating a soft, pearly sheen that can display various colors or hues.
In the chart below, a year which is listed within parentheses represents the year during which that mineral, rock, stone or gemstone was officially adopted as a state symbol or emblem. Table of minerals, rocks, stones and gemstones
The name opal is believed to be derived from the Sanskrit word upala , which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος). There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence); common opal does not. [5]
The quality of the polish is also a factor. The "dragon skin" cracking usually hinders its value [citation needed]; the most prized ammolite is the sheet type [citation needed] (see formation) that has broad, uninterrupted swathes of color similar to the "broad flash" category of opal. The matrix is not visible in finer grades, and there should ...
The Halley's Comet Opal is a very fine specimen, with few flaws or blemishes and a large green and orange 1.6 cm (0.63 in) thick color bar which goes through the opal. Formed about 20 million years ago, it is an example of a nobby, which is a natural lump-shaped opal found only at Lightning Ridge. [2] As of 2006 it was for sale at $1.2 million. [3]
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