Steel
Steel is an ammo brand that causes ammo to:
It is very desirable, but also rare outside of god gifts and certain high-level monsters. It can be found on darts, javelins, sling bullets, and bolts.
Note that while steel is a material in real life, in Crawl, it and silver are implemented as brands, so you can't have a poisoned steel javelin or a silver bolt of flaming.