Lapidaryforum.net
Let's Rock => Identifications of Materials => Topic started by: jsoukup on February 10, 2019, 10:18:12 AM
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Found this in an alluvial stream in southeastern South Dakota full of glacial till so it could have originated anywhere north and west of there.
Makes a good paper weight but I would really like to know what it is.
The rind looks and feels like mortar and the shiny green and black areas feel like glass.
Thanks
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Hardness and streak?
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I do not have a clue what that rock is. It is pretty.
Welcome to the forum!!
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The fracture eliminates Jade and Serpentine. If a 7 MOH's it is some version of agate and if closer to a MOH's 5 probably a version of opal.
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It looks like a sedimentary chert nodule to me.
https://www.google.com/search?q=chert+nodule+images&tbm=isch&source=hp&sa=X&ved=2ahUKEwiwmu6ymbLgAhVlHDQIHSPXBaAQsAR6BAgDEAE&biw=960&bih=420#imgrc=lRkyDXGexvIReM:&spf=1549837255326
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scratched pretty hard with a toenail clippers tried it on a brandy bottle to compare and the glass bottle scratched way easier
does have a chertish look but coloring is closer to agate I would say
I'm gonna keep telling my daughters is kryptonite - I keep it around because I'm scared of flying
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i am betting petrified wood just by seeing the center up close
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Whatever it is, it's really pretty stuff! Welcome to the forum!
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Given the hardness and shell, I suspect peridot. If so, it would yeild some extremely large gems.
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A peridot bomb ? could be.
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Given the hardness and shell, I suspect peridot.
A peridot bomb ? could be.
According to Mindat, olivine group minerals do occur in South Dakota (Custer and Perkins counties, at the very least), so a mix of olivines (including peridot) is definitely a possibility.
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Something petrified has merit does everyone agree that it was a void filled in a sedimentary layer? Looks like it may have been a vein at one time to me.
When I first started researching it I really though it was obsidian because it is very opaque and not really any crystal growth but the rind rules that out.
The attached pictures are to support the vein theory of mine.
Again the rind looks like fused sand.
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The color does look like "Olivine (Peridot) Forsterite" perhaps it has a shell weathered by the stream?
The black and gray in the center portion kind of throws me off with that theory, is that something anyone has seen?
Now you got me all exited under normal light it looks pretty plain. Under a magnifying glass and in pictures its very beautiful. Is it possible that its more translucent than I thought because the light can't go all the way through it making it look black opaque? When I shine a light in it no light comes out leaving me to believe it was opaque or may that not be the case?
the prettiest part looks like an old black onyx ring my dad has, the rock could be a movie star...pretty in pictures and less impressive in real life
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The olivine group has a wide range of colors, from colorless to near-black with shades of yellow, green, gray, brownish-reds and in-between. Aside from the color and rind, one reason I suspected olivine is the lack of conchoidal fracture on the broken end (which would stand out if it were agate, jasper or a silicified petrified wood).
Olivines (and peridotite mixes) eventually can develop crusts/rinds on the outer surfaces due to weathering alteration (from air, water or chemical reaction with adjacent material), so you are correct that this may be the explanation for the exterior of your piece.
You might want to use Mindat's map of South Dakota (https://www.mindat.org/loc-4104.html) to zoom in on the general region where you found this to see whether there may be other possibilities reported from that area.
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Thanks everyone for looking at it.