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For Painshill
Painshill Wrote:
Back in the seventies I was told Tabletop chert was a Jasper Kremmling chert was a chalcedony. and from an Archaeologist that Trout creek chert referred to as Jasper as Pet palm wood, The only two things that has not changed. Is Quartzite and Obsidian. I can understand why from the granulation from Quartzite Like the Hixton and Obsidian being from of a different method of volcanic but with the same refractory elements. I do like lithic's and origin of deposits. But what has confused me what makes them separate. I know that all are silicon based material. and sands depending on the region can be coarse to fine and mineral induction will effect color. But to use terms loosely, I just want to get the terms right.
All of these things are in the quartz family. Quartz is a mineral not a rock, since it always has the same chemical composition… silicon dioxide. The variants which quartz produces are distinguished by the way in which they have crystallized and the impurities or inclusions which are present. Those things in turn influence the colour and appearance. In this case, we are talking about “cryptocrystalline” quartz. That is, the crystals are so small that there is no apparent crystalline pattern… even under high magnification.
Although, as USGS says, “chalcedony” is a catch-all term we can be a little more precise than that. Chalcedony is a rock not a mineral. It’s composed of intergrowths of two different types of quartz: cryptocrystalline silicon dioxide plus moganite. Pure quartz is colourless and pure moganite is grey but chalcedony can be pretty much any colour, arising from mineral impurities. Yellow, brown and red tints are usually the result of small amounts of iron compounds being present.
As we move further away from that level of purity and towards more complex methods of crystallization the variations of chalcedony are given specific names which vary from geologically acceptable lapidary terms (eg carnelian) though to ridiculously fanciful new age terms (eg Tibetan snow cloud stone).
The generally accepted conventions are that when chalcedony has multi-coloured bands it’s termed an agate. If the banding is black and white we call it onyx… and so on. When it’s not banded but is still clear or translucent and between orange to deep red we call it carnelian. The brown variant of carnelian is called sard… and so on. Tibetan snow cloud stone? Don’t even ask!
When there are higher levels of impurities and the particles are large enough to make the stone opaque, we call it jasper, which is commonly red from iron (III) inclusions but can also be yellow, brown or green and occasionally blue.
Many of these terms used are not properly defined geologically and have come from the gem and lapidary trade, but are nevertheless tolerated (to varying degrees) despite being heavily mis-used by non-geologists.
Hope that helps a bit.
Although, as USGS says, “chalcedony” is a catch-all term we can be a little more precise than that. Chalcedony is a rock not a mineral. It’s composed of intergrowths of two different types of quartz: cryptocrystalline silicon dioxide plus moganite. Pure quartz is colourless and pure moganite is grey but chalcedony can be pretty much any colour, arising from mineral impurities. Yellow, brown and red tints are usually the result of small amounts of iron compounds being present.
As we move further away from that level of purity and towards more complex methods of crystallization the variations of chalcedony are given specific names which vary from geologically acceptable lapidary terms (eg carnelian) though to ridiculously fanciful new age terms (eg Tibetan snow cloud stone).
The generally accepted conventions are that when chalcedony has multi-coloured bands it’s termed an agate. If the banding is black and white we call it onyx… and so on. When it’s not banded but is still clear or translucent and between orange to deep red we call it carnelian. The brown variant of carnelian is called sard… and so on. Tibetan snow cloud stone? Don’t even ask!
When there are higher levels of impurities and the particles are large enough to make the stone opaque, we call it jasper, which is commonly red from iron (III) inclusions but can also be yellow, brown or green and occasionally blue.
Many of these terms used are not properly defined geologically and have come from the gem and lapidary trade, but are nevertheless tolerated (to varying degrees) despite being heavily mis-used by non-geologists.
Hope that helps a bit.
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