From Dragon Eye Atlas

A rare metal that is both light and strong, most famously used for armour.

Mithril ore is about as common as silver, but it is much harder to refine into pure mithril, and requires very high temperatures, which are hard to accomplish outside a few volcano-fueled dwarven forges. Rumours have it that only mithril smelted by dragonfire is of the highest purity. At most one ton of such mithril exists in the world, according to some researchers, left from ancient times.

Mithril is also difficult to work with as it has a high melting point and becomes brittle if not cooled properly. Its rarity and difficulty of crafting make mithril armour very expensive. Additionally, the forging and working of mithril is a dwarven secret that no human has discovered yet, and that the dwarves guard carefully.

A chain mail of mithril is no more cumbersome than a set of heavy cloth, but as solid as the best steel. It would cost at least 50 times the price of an ordinary chain mail, if it were available on the free market at all. It is much more likely that it would be a present to a king than that it were on sale anywhere.