Obsidian

Obsidian, also called dragonglass, is a common mineral around the center of the world, and is also found on the dragon-infested continent of Magithea. It is prized for its hardness and its tendency to fracture into razor-sharp pointed pieces ideal for arrowheads or spear-tips. It can also be, with some skill, crafted into a sword. It is commonly believed that obsidian is the only substance that can pierce dragonhide.

Creation and availability
Obsidian can be created in one of two ways. The first is by volcanoes, and other sources of earth-fire. Magma, when cooled rapidly, may turn into obsidian, which results in many strange landscapes such as those found on Magithea. The second is, of course, dragons. Many fire-breathing dragons can emit flames hot enough to melt stone, which often cools into obsidian shards, especially near sources of water. Because of its rarity, it is often extremely valuable, and a single shard of dragonglass may be valuable enough to buy you a horse.