This is a piece of Trinitite. This particular piece is from Alamogordo, New Mexico. Trinitite, all of it, was formed in a split-second when the first nuclear bomb exploded on July 16, 1945. This piece is slightly greenish, smooth on the top, and rough on the bottom. Enclosed on epoxy and labeled Trinitite & Los Alamos & Scientific & Laboratory this artifact must have sat on a desk.
Here's more (a wiki article on Alamogordo glass, a nice article on it, and another with some for sale).
Update: I emailed Los Alamos National Laboratory and asked. No reply yet.