Dwarf Steel

Though this light and strong substance can be found around the world, only dwarves have successfully smelted it and used it to create potent weapons and armor. 'Dwarf steel' refers to two things, both the mined ore and the finished product; the latter is considerably stronger than standard steel, and myriad swords and spears of legend are said to be dwarven craftsmanship.
Not only is the secret of smelting this material kept fervently by dwarves, the art of working it into any sort of usable material takes years of practice, and decades more to perfect. "Even if I gave you the recipe, you'd have no idea how to work with it" most smiths claim, and it's true. Adding the right amount of unalloyed dwarf steel to an unfinished product takes a keen eye and an almost supernatural awareness of one's art, and the dwarf steel ore won't serve as well on its own. The results are worth it however, with contemporary tales recounting how dwarf steel swords cleaved lesser blades with nary a scratch, and millennia old blades persisting with nearly no rust or loss of efficacy. More than any other marvel they've perfected, this product is the hottest commodity of dwarves when it comes to trade, and they jealously portion out even the raw ore to others (though if anyone has successfully used the ore, it's only the elves, and they've yet to reveal as much).
Though mining itself is a job that most dwarves view as necessary if not particularly demanding in terms of skill (though the engineering prowess of foremen is a respected craft), smelting special ores is held in the same high regard as smithing. Though other metals are used in the dwarves' best steels (and even most dwarves don't know where the House who manufactures Brightsteel gets their secret ingredient), dwarves learned long ago that letting humans focus on one material would make their opacity simpler to maintain.