Exsidian was a metal ore. An especially durable metal, exsidian suffered much less rust and erosion from exposure to the desert climate than other metals did. Due to this property, it was highly prized by Glatorian and Agori alike as plating for their weaponry or tools. The village of Iconox's supply of exsidian allowed it to thrive in trade.
Glatorian fortunate enough to obtain some exsidian brought it to Vulcanus to have it melted down and used as plating for their weapons.
Exsidian was a metal that resisted wear and corrosion very well (a constant problem in a desert), so it was prized for use as a blade coating.