Obsidian

Obsidian (OB) is a reagent that is in use on Pagan.

Pagan
This dark stone, while often used for decorative uses, has proven to to be very useful in the arcane arts as well. For the magic school of Sorcery, obsidian proved to be an ideal reagent to add duration to a spell, since due to its nature, it can endure the heat of a volcano for a prolonged time without much ill effect. This is little wonder, as it is created from fast cooling lava that has only a small water content in it.

While the currency of Pagan, Obsidians, are also minted from the same material, the coins are useless for spellcasting purposes, as a special heating and molding treatment - which is a government secret - destroys the properties that make obsidian a reagent.

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.