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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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<blockquote data-quote="user 4386" data-source="post: 403353" data-attributes="member: 4386"><p>It is definitely opaline silica and probable chalcedony (microquartz). If from, say, the ridge north through Coober Pedy, or Stuart Creek etc be wary. Many of the opalfields in that area are overlain by a silica rock called silcrete, which is much more continuous over wide areas. The silica in it looks a lot like this - in fact one is a dead ringer. I am not sure that silcrete is a direct indicator of underlying opal though, In a similar setting at White Cliffs I have seen opal broken up into old gravels and only then covered with silcrete, suggesting that they did not form at the same time. Silcret often forms right at surface and often has silica fragments cemented by silica (a silica breccia).</p></blockquote><p></p>
[QUOTE="user 4386, post: 403353, member: 4386"] It is definitely opaline silica and probable chalcedony (microquartz). If from, say, the ridge north through Coober Pedy, or Stuart Creek etc be wary. Many of the opalfields in that area are overlain by a silica rock called silcrete, which is much more continuous over wide areas. The silica in it looks a lot like this - in fact one is a dead ringer. I am not sure that silcrete is a direct indicator of underlying opal though, In a similar setting at White Cliffs I have seen opal broken up into old gravels and only then covered with silcrete, suggesting that they did not form at the same time. Silcret often forms right at surface and often has silica fragments cemented by silica (a silica breccia). [/QUOTE]
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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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