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What is this greenish colored rock?

0 votes

I received this as a gift so I have no background information on this piece. When I felt it, the texture felt the same as a rough piece of calcite. But when I googled "green calcite" as a guess to compare, it shows up much greener than this piece so I am now stumped. Any help is appreciated.

asked Aug 30, 2016 by JohnBagel (1,010 points)

1 Answer

+1 vote
This also looks like quartz with some staining from other minerals.
answered Aug 30, 2016 by Weasel (38,360 points)
Thank you, any possible guesses as to what it could have stained with? I'm a novice and you seem to be quite the expert. You'd give a better guess what I would.
You can make a better guess by trying to scratch it with a nail or a pocketknife. If it is quartz nothing will scrape it short of a harder substance. If it is calcite it would scratch with the nail. Any mineral could stain it. Quartz is very hard but the larger molecules allow the minerals to stain it readily. Red and brown and yellow from iron. Blue from titanium, green maybe copper.  Give it a scratch and let us know.
I took a pocketknife to it and it did slightly scratch the surface. I didn't try with a nail. Could it still be calcite even by scratching with the knife?
*Referring to my previous question about the quartz, I took the knife to that as well and that did not scratch. So that is presumably quartz because it did not scratch?
Yes, you would not be able to scratch quartz, so it is not quartz! If it is calcite you should be able to drop a weak acid like vinegar on the surface and it will bubble. A streak test would yield a white streak. Always work in a well ventilated area.
I would like to try the vinegar test to see if it could be calcite, but I'd rather not have it get bubbly. Too bad a piece hadn't broke off. What exactly is a streak test? How would I go about doing that, what do I look for, etc?
You could rinse it off almost as soon as it starts and the gas released by this test would be carbon dioxide. A streak test you can google but it is simple as rubbing a ceramic tile with the substance and it will leave a steak, in this case it would leave a white streak.
Thank you again for all the help and information. I will consider the vinegar test, I just like to keep the pieces of my collection in the same shape as when I got them, so making the surface bubbly makes me leery. But if it would confirm what it is, then it would be worth it. I will look for a ceramic tile and try the streak test and update you whenever I get around to finding one.
Hi Weasel, so I got a ceramic tile and did a streak test. A white powder came off afterward so as you said it would for calcite. What I'm not sure of right now is I found a chart for the Mohs hardness scale and it showed that a penny could scratch calcite. So I took a penny to this piece but it didn't do anything. Yet it did scratch with the knife. So could this piece be calcite with a matrix of something else?
Sorry, I would like to edit my last statement. I tried using the nail again and noticed it did scratch the surface, but only on the green parts. It would not scratch the white. I also tried using a screw, fingernail, and penny and they too could only scratch the green parts and not the white. Could the green be calcite and the white be quartz?
 


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