Physical Properties of Opal Gemstone Opal is a gemstone that is famous for its internal glowing color, which takes on myriad hues and shades, rather like smoldering flames in a block of ice. Opal gemstone crystallizes in the form of masses and is not a very hard stone (only 5.5 – 6 on the Mohs scale), but is still used as jewelry due to its striking color variations, of which there are many. Its color is caused by its unique internal composition and structure – it is a hydrated form of silica (SiO2·nH2O) essentially making it a “watery stone”, and
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