Dictionary of Ichthyology  

glass float = a hollow ball of glass once used by fishermen to support nets in various parts of the world, now replaced with modern materials such as aluminium and plastics such as styrofoam. Most numerous in the Pacific from Japanese fishing gear, still washing up on shore and now collectibles. The Japanese ones are mostly greenish from the long exposure of recycled sake bottles. Norwegian glass floats were egg-sized and used with hook and line. Also called glass fishing float and Japanese fishing float.

Japanese glass fishing float with a net, 14 inches in diameter from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:Large_Glass_

Canadian Museum of Nature