What crystalline mineral is this echinoid test made of?

[TylerD]

I found this Coenholectypus echinoid in the Mainstreet Limestone in Fort Worth, Texas. Unlike the heart-shaped echinoid that I've found in other formations, these type echinoids' tests appear to have fossilized into a dark crystalline mineral. I'm not quite sure what mineral this is, and hope that it doesn't react to acids so that it can be acid prepped. It can be hard to see in the image, but on the edge of the fractured test you can see that it's made of a dark mineral with a reflective shimmer. I'm not sure if it is just calcite with impurities to make it that color, but of the 5 specimens of this genus, the 3 that are also crystalline have the same coloration. 

[Ludwigia]

I have the inpression that you are dealing with calcite here, so I would recommend you not use any acid on it. Potassium hydroxide flakes with a few drops of water will do the trick. See this topic for an idea of how to do it best.

[JohnJ]

I agree with Roger that the test is calcite based.  The darker color is likely due to impurities from the decomposition of pyrite within the test.  

 

I wouldn't use acids or KOH on that specimen.  It is too fractured to tolerate the residual effects of either.  Gentle brushing with a damp, stiff toothbrush should help some.  Otherwise, call it good and go find a better one.