http://www.personal.psu.edu/its5014/...mons/stone.jpg
Well, the last "mystery stone" I bought on eBay got refunded (seller had lost it)... so I felt the need to buy another. Here's the pics, any guesses as to its type? You've got a week or so before it should get here thanks to the holiday, so have at it.
The auction didn't list stone dimensions but the box is 9.8"x3" so I'd estimate the stone is right around 8.5"x1.5".