These are Eleven Little Black Squares that were cut out of black acrylic to make the holes in Volcano Boards... so they fit perfectly back in again. They also fit into the all-white Pyramid Arcade Volcano Board.
Eleven is the number you need to convert a 6x6 MegaVolcano board into a regular 5x5, so we called the pack of eleven of them ELBS (Eleven Little Black Squares) thus we call a single square an elb (this is a type of word derivation known as back-formation).
They're also just fun to play with: block off certain spots, or even make a custom shaped board! (peel off the protective brown paper to see their beautiful glossy blackness.)