Lava is molten rock expelled by a volcano during an eruption. One constituent of the lava flow is pumice, or volcanic glass. It is commonly light gray to blackish-gray in color. Holes form as gases froth the molten rock, which remain as it cools. This characteristic also makes the stone extremely lightweight. Lava beads mix well with any gemstone and come in round, cube, flattened disc, oval and pillow shapes.