A highly reticulated type of rock formed in volcanic eruptions, looking somewhat like steel wool.
Magma contains small amounts of gas; when magma is forced to cool quickly, this gas may expand, but still remain trapped in the rock, forming bubbles. This usually forms pumice, with 64%-94% of its volume composed of gas bubbles (meaning that many samples of pumice will float in water). If the gas bubbles compose more than 95% of the rocks' volume, the walls of the bubbles may burst and leave an open reticule, thin strands of volcanic rock (actually classified as a form of glass, as it is not crystallized) connected in a open framework.
Reticulite is formed in volcanic eruptions in which there is a high plume of ejecta, throwing hot magma high into the air where it cools quickly (this sort of eruption is called strombolian). Reticulite completes its formation while still in the air, and is light enough to be carried away by the wind, perhaps traveling airborne for kilometers. As you might expect, it is also very fragile, and does not last long unless it lands somewhere protected. Because it is an open framework of glass strands reticulite will not float in water, although it is the lightest known type of rock. Reticulite is usually gray or black in color, but you may also find golden reticulite.
Reticulite was originally called thread-lace scoria, although it is not usually very mafic, meaning that most samples are not technically scoria, but a type of pumice.
http://volcanoes.usgs.gov/Products/Pglossary/reticulite.html (Good photo at this site)