I came into possession of a bunch of little display squares of flooring stone when a flooring display showroom closed. Each one was about 75mm square and they were attached to display boards. Ripping them off the boards and printing out this holder makes a nifty coaster and conversation piece.

The model has an option for making holes in the bottom into which you can put rubber or cork to make little feet.

It's an OpenSCAD model so works with the customizer. You can set the overall coaster size, as well as the inset space for the stone. You can also optionally add four small circles on the bottom corners suitable for adding rubber or cork feet.