What is this slice of rock ?

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Offline catherine

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What is this slice of rock ?
« on: 08/04/2005 03:23:53 »
Can anyone help me to identify this slice?  I would like to buy many of them but do not know what it is.  The diameter is 25mm, and it is 3 mm thick.  I bought it in Tucson from a south african, but don't know if that is where it is from.  any help would be appreciated.  Thanks!



Offline Bass

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Re: What is this slice of rock ?
« Reply #1 on: 08/04/2005 19:02:09 »
The photo appears to be a slice of a geode (actually a nodule, since it is completely filled).  These are round voids in rocks (common in desert environments and certain volcanic rocks) that fill with minerals, usually silica (quartz, agate, jasper, opal), calcite, flourite, amethysts, etc.  The crystals grow from the outside toward the center.  I would guess that your slice is probably agate, but I can't tell for sure from the photo- the concentric bands are growth rings.  The different colors are from impurities in the rings.  Can you see any crystals in any of the bands?  Is it harder than a steel knife (will the knife leave a scratch on surface)?  Try scratching a small area where it won't be noticed, if you think it scratches the surface make sure the mineral is scratched and you are not looking at steel scraped off the knife.  I suspect that you won't be able to scratch this specimen, which means it is probably agate.  If it scratches easily, it is probably calcite.

Prediction is difficult, especially the future.  -Niels Bohr
« Last Edit: 09/04/2005 01:51:55 by Bass »
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