We all know that adamantine is the best metal so far, usually because of it's ease of access and high qualities.
But what if there was there was an even higher quality of metal that was rare and hard to get without going to extra lengths to get it?
The idea I present to you is rainbow gold, and ghost silver.
In this idea, Unicorn horns are made of rainbow gold. A single horn, when melted down, yields a varying amount of bars.
The quality of the metal depends on how old the unicorn is. 100 year old unicorns should yield metal that is maybe half as good as adamantine, while 200 year old unicorns should yield metal that is just as good, with older unicorns yielding the best quality.
Ghost silver comes from the bones of undead that have been around for a while. when an Undead has been around for at least 10 years, their bones start hardening into ghost silver.
Like with Unicorns, the older the undead, the higher quality the bones are.
What do you guys think?
Edit: obviously, Unicorns would have to be altered to make this an actually rare thing. Ideally, once a unicorn reaches 10 years of age they become legendary in all combat skills, and are untamable from birth, have a preternatural ability to dodge arrows, and become hostile to dwarves when their young are threatened.
Just throwing some ideas out there.