What would you use a dozen 4" x 3" glass pieces for?

If it's clear glass, you can make mushroom spoor prints on them. Print on one piece, then seal a second piece around the edges to preserve the print in between the two panes. One of those Portobello mushrooms you can find at the store would make a great one to start with.

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Paint them with glass paints and put a frame around to make either a window or picture.

If you have glass cutting tools, there are many things you could do.

Otherwise, you could make a hexagonal box with six divisions, kind of like this one:

http://digiplayground.com/images/P/Large-2pc-Hexag...

You can glue it to a base made out of plexiglass for an all-glass effect.

they can make perfect paper weights or glass rests with plastic glued onto the under side as embellishments multipurpose as the foot rests.

If they were different colors, you could make a mosaic for a window.