Jeffrey St. Clair, Ph.D. wrote:
Are you sure it isn't slate? They look pretty similar as they're
geologically related. I do better with pictures so I thought I'd share one
that has a slate plate (lol) in the tank.

http://www.flickr.com/photos/ifnar/1...7601124618440/

Other than the color, it looks the same as in your pic. Mine is dark
gray and extremely dense. It looks more like solid rock as opposed to a
sandstone sort of look. It's rather heavy too. Time to hit the books
to better ID it.

Grandpa John