Debbie Mumm's Easy Breezy Mosaic Garden Accessories include ideas for a birdhouse and flowerpot that use sea glass in their designs.
Perhaps a plastic organizing bin or a decorative flowerpot would be more fitting.
A tree branch placed in a large flowerpot filled with polished river rocks makes a great display area for ornaments as well.
A clay flowerpot cut in half makes a good sleeping area.
Looks like a chunk off an old flowerpot to me.