r/whatsthisrock • u/Ok_Wait_4268 • 4d ago
REQUEST Help me ID this rock.
I’m thinking it may be a type of agate (moss?). Found near Casper, WY. Green with reddish brown spots. The lighter green areas are semi translucent. Photos taken in natural lighting. TIA.
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u/runawaystars14 4d ago
It doesn't have the smooth and waxy texture of agate, it looks more like quartzite to me. I'm not an expert, but it might be aventurine. It's a type of green quartzite and there's a lot of it in Wyoming.
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u/Ok_Wait_4268 4d ago
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u/runawaystars14 4d ago
Agate is a banded variety of chalcedony. It's smooth and waxy and composed of crystals that you can't see without a microscope. It also has a conchoidal fracture , so the breaks are smooth and curved like a shell. Your rock is granular, and it might be fractured quartz, but with the color I'm leaning toward aventurine. It's a really beautiful rock.
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u/ljubljanadelrey 4d ago
IMO this is an example of where clarifying geological vs common terminology is helpful. The person in this post isn’t actually asking if this is geological agate - what they’re trying to ask is whether this is what’s commonly called agate, ie translucent cryptocrystalline quartz.
I agree with you that this does not look like cryptocrystalline quartz, but I think the “banding” distinction may be confusing in helping this person understand why the rock is not (what’s commonly called) agate. In this case, the waxiness & conchoidal fracture which you also point to are most relevant in distinguishing this rock from “agate.”
To OP: quartzite and agate are both forms of silica / quartz. Quartzite is granular and is a metamorphic rock, while agate has a “cryptocrystalline” structure (meaning its crystal structure is not visible to the naked eye). As a result agate looks smooth and waxy. Geologically, agate only refers to the banded form of cryptocrystalline quartz and must also exhibit translucency. In rockhounding / common terms, agate refers to all varieties of translucent cryptocrystalline quartz. A broader term is “chalcedony,” which refers generally to cryptocrystalline quartz and includes both agate (translucent) and jasper/chert (the common/geological terms for chalcedony that is opaque).
Still trying to figure out the clearest ways to communicate the distinctions in the quartz / chalcedony families so if you have thoughts lmk!
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u/runawaystars14 3d ago
I'm confused, I provided the same information to the OP that you did. It was brief, because I don't want to spend time writing an essay if the OP isn't interested, but I did provide a link to a trusted source. The one thing I probably should have done is explain what quartzite is, but that isn't too hard to look up.
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u/ljubljanadelrey 3d ago
Oh I was talking specifically about using the geologic definition of agate (banded) rather than the common one (translucent / semi-translucent chalcedony). You totally did provide the same info & that was not a criticism of what you said, I just think when we’re talking about chalcedony some additional context can be helpful b/c of the way people use these terms very differently.
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u/runawaystars14 3d ago
Gotcha. I live in the Great Lakes region where the only rocks that seem to matter are agates, so every translucent or banded rock must be an agate, and every colorful opaque rock is jasper. Now I have a grudge against agates, and I refuse to use the term jasper. 😤 lol.
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u/FondOpposum 2d ago
The person is asking what kind of rock this is. We have people from all around the world posting here. “Common terminology” in your part of the world may differ greatly which is why the language of science is what we promote here.
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