Pumice I picked up on the beach within walking distance of the ruins of Akrotiri on the island of Thera in spring 2005. The black lava sinks; the red lava is bubbly enough that it sinks very slowly, like waterlogged wood; the white pumice is so full of holes it floats.

© 2005, Ellen Brundige. Creative Commons License: you may use this photo for non-commercial purposes if you give a photo credit with a link.