Vitrean Steel

Vitrean steel, sometimes known as "vitreus" or "metal glass," is a type of ancient and near mythological material used to create weaponry with unique properties. It was discovered by the Kroscalan Empire and used in their weapons and armor, and was common in the Fourth Age before the Astral War. The Yezeth people also had access to vitrean steel, though whether they could manufacture it themselves or traded with Kroscola isn't known. By the Fifth Age, the knowledge on how to create vitrean steel has been lost.
Vitrean steel holds an edge even sharper than obsidian, and it is highly resistant to corrosion.  It is significantly stronger than normal steel, but enough damage can cause materials made from it to shatter suddenly and unexpectedly, like glass.