Pumice

Pumice (PU) is a reagent that is in use on Pagan.

Pagan
This brittle stone, while not very useful for other uses, has revealed itself to to be very useful in the arcane arts. For the magic school of Sorcery, pumice proved to be an ideal reagent to add distance to a spell, since due to its nature, it had been thrown the longest way out of a volcano, thanks to its very low weight. As the longest-thrown stone, it is essential for offensive spells that are fired at enemies.

Trivia

 * Technically spoken, obsidian and pumice are almost the same, the difference just being the amount of water and the time it takes the lava to cool down.
 * Pumice is also found on the Serpent Isle, where a teleport storm exchanged a bit of pumice from Furnace with the Avatar's spellbook.