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Gemstones, Minerals & Fossils
Gemstones and Minerals
North Wollongong Beach fossicking
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<blockquote data-quote="user 4386" data-source="post: 664696" data-attributes="member: 4386"><p>The wood might be associated with the coal, although often it comes from young gravels, often beneath basalt flows. The original wood has gone of course, but each cell in the wood has been perfectly replaced by silica that has crystallized from low temperature water flowing through an aquifer (e.g gravels beneath impermeable overlying basalt).</p><p></p><p>Agates can have varying origins - again it is silica (chalcedony) precipitated from water but it usually fills an open cavity that is often present already, commonly but not always in a lava. There are both low temperature and high temperature varieties, the latter a bit like thunder eggs and formed from hot water in the cooling volcanic rock (that may be derived from it, rather than travelling through it from elsewhere).</p></blockquote><p></p>
[QUOTE="user 4386, post: 664696, member: 4386"] The wood might be associated with the coal, although often it comes from young gravels, often beneath basalt flows. The original wood has gone of course, but each cell in the wood has been perfectly replaced by silica that has crystallized from low temperature water flowing through an aquifer (e.g gravels beneath impermeable overlying basalt). Agates can have varying origins - again it is silica (chalcedony) precipitated from water but it usually fills an open cavity that is often present already, commonly but not always in a lava. There are both low temperature and high temperature varieties, the latter a bit like thunder eggs and formed from hot water in the cooling volcanic rock (that may be derived from it, rather than travelling through it from elsewhere). [/QUOTE]
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Gemstones, Minerals & Fossils
Gemstones and Minerals
North Wollongong Beach fossicking
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