Identity Helpdiamond shaped crystals on chalcedony
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if im not mistaken the yellowish stuff is calcite and has been dissolving and exposing the crystals. any ideas on what they could be? its from western MT.
It looks like calcite rhombs either replaced by chalcedony or coated by chalcedony, Did you do a scratch hardness test? Corroding calcite exposing bluish botryoidal chalcedony. Look closely near where the bluish chalcedony goes into the dark area and you can see the distinctly botryoidal surface. Put a few drops of acid onto the yellowish areas; there should be vigorous fizzing after several seconds to confirm the identification.
As an addendum, in my opinion, the diamond shaped areas are all part of the same chalcedony. looking closely at the bluish areas as a whole, suggests other areas closely resembling the diamond shaped areas. CHEERS.....BOB The "diamond shapes" look more like gypsum forms to me--could be chalcedony-coated gypsum crystals. Interesting. 
I agree with Ed. If any gypsum is left it won't fizz in acid. thanks for the answers. if they are calcite rhombs that would mean they broke cleanly into that shape. correct? do calcite crystals grow into a rhomb? I get the feeling they are individual crystals grown there in place. I can try a scratch test in case they are gypsum. of course if they've been replaced it wont tell much. I have seen feldspar with similar shapes at certain angles but I don't know if that would be possible in this kind of environment. I guess I was hoping someone would know immediately just from the picture. are gypsum crystals common on chalcedony? Gypsum happens but Bob would have a better idea how common it is. I found very similar calcite rhombs in mindat photos from Washington. not sure how to add the photo here though. so I am leaning toward that, replaced by chalcedony. I did find selenite photos found in geodes but they didn't match the shape as well as the calcite. thanks again Gypsum is a thought due to their overall shape, but I find it difficult to believe that gypsum, being so soft, would corrode to those rounded shaped crystals without breaking or cleaving etc. And the grayish color and the dark crevices in them (especially the top one) just doesn't look right to me. If calcite, add the few drops of acid to both the yellow areas and those diamond shaped crystals. I still think the best bet is all chalcedony under the corroding calcite. CHEERS.....BOB a drop of acid rolled past the diamond (with no response) to the calcite where it fizzed. pocket knife wouldn't scratch the crystal. I guess that rules them out as calcite or gypsum. must be chalcedony pseudomorphs after calcite or gypsum.i may cut the crystal in half and then add acid in case there is calcite still inside it and maybe soak the whole thing in acid to what else is in there.
. A thick chalcedony coating on a sharp-edged and sharp-cornered gypsum crystal will be rounded, no corrosion required. And a chalcedony coating (if that's what it is) won't fizz in acid no matter what is or was inside it.
Barite also frequently forms diamond-shaped tabular crystals and so might be another candidate for the original mineral. thanks Ed, how to know? either calcite, gypsum or barite. You'll have to break one open and hope there's some of the original mineral left. If none then you have the typical pseudo problem- What was it??? 
I've found a lot of chalcedony after calcite crystals in geodes from Saguache, Colorado. I believe like others that you have chalcedony after calcite.
Opening one of the crystals could reveal what carbonate you have, but if its like mine, then they'll be hollow with no remaining calcite. Perhaps I should label my specimens as chalcedony after carbonate as I cannot prove it was calcite. But one thing I can say is that it was probably some form of carbonate due to the rhombic shape.
Chalcedony after gypsum isn't all that uncommon, but I suspect instead often overlooked because (a) the shape may be ambiguous, and/or (b) they tend to be rather ugly. just a quick follow up. I did soak the whole thing in acid and the top separated from the bottom. the bottom is covered in calcite casts. the top half is only chalcedony.. one interesting thing is there is one sharp edged blue crystal below the white diamond shapes. not sure why.
I suppose they must be calcite/carbonate crystals because they are hollow. barite and gypsum wouldn't dissolve away so easily. thanks for the helpful thoughts.
I suppose they must be calcite/carbonate crystals because they are hollow. barite and gypsum wouldn't dissolve away so easily. thanks for the helpful thoughts.
