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for whatever reason, I didn't even notice the black stone. I thought it was a box!

Its probably not black either, just layered an a whole load of grunge. Likely a Washita or the likes.

My basic thoughts on the stone... As stated by all above, Translucent, and a great stone.
If its already flat, leave it alone. If you need to flatten it, you are in for a whole lot of exercise!...