Originally Posted by lochaber
I think you found the right wiki page for it.
I wouldn't stick it in an aquarium. gypsum won't hold up to water, and even if it doesn't affect your water pH/hardness, it will breakdown/dissolve. And that looks like a pretty nice sized piece. It would be a shame to see it damaged/destroyed.
Yeah, that's kind of what I was thinking. That piece is probably about 5 inches across. It's the biggest one in my collection. I have chunks down to the size of my thumbnail. Looks like I'll just be cleaning them up and displaying them instead!