Limu o Pele is Hawaiian for 'Pele's seaweed'. Just as drops of obsidian are called Pele's tears, and fine windblown strands of obsidian are called Pele's hair, these glass fragments formed from obsidian bubbles are called Pele's seaweed. For whatever reason, limu o Pele is not usually translated into English, although Pele's tears and Pele's hair almost always are. Instead, this term is always used in the original Hawaiian. The English term 'lava bubble fragments' may be used by volcanologists with insufficient poetry in their souls.
These bubbles are formed when water gets trapped in hot, rapidly flowing lava (generally when a lava flow pours into the sea), and turns instantly to steam. The steam inflates the bubble, which quickly cools, hardens, and bursts. The limu o Pele is often light enough to be carried by the wind, and collects in cracks in the rocks on the beaches. It looks like fragments of green, brown, yellow, or bright golden glass, but very thin glass -- the glass you get from breaking a light bulb rather than from a window. There may be smaller bubbles visible in the glass fragments.
Limu o Pele has also been found around deep-water volcanoes, where the pressure would be too great to allow expanding steam to form the bubbles; in these cases, it is assumed that CO2 gas from the volcanic vent inflated the bubbles.
References:
http://volcano.und.edu/vwdocs/vwlessons/tephra/part10.html
http://www.agu.org/meetings/cc02aabstracts/Maicher-deepsea.pdf
Photos:
http://volcano.und.edu/vwdocs/vwlessons/tephra/part10.html (Various forms of volcanic glass)
http://www.mbari.org/volcanism/Ridge/R-ExplosiveErupt.htm (A foraminifera that has collected limu o Pele to decorate its shell).