Yowrah nut cabochon These beautiful Australian opal nuts consist veins of precious glowing opal within nodules of siliceous ironstone that often form amazing patterns. They vary from 0.5 to 20 cm across, and occur in an iron rich sandstone, near the border with an adjoining mudstone. This one has been sliced and polished into cabochons, revealing the intricacy of the patterns of the gemmy silica within. Loz Image credit: J.J. Harrison
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