Our sister site, Engadget, has a post
about a coaster project from Evil Mad Scientist Labs
These coasters are made from the photomasks used in semiconductor manufacturing; the patterns on the photomasks are etched into the components in the factory. The masks are solid glass with a thin metal design applied to them. You can get them from retailers who deal in electronics
Be sure to check out the comments on the Engadget
post, where readers discuss safety
issues and sources for the materials. It's useful to know, for example, that shattered quartz glass is unusually sharp -- and difficult to remove from a cut. The reader suggests backing the coasters with clear plastic; I suggest keeping them away from small children.