Nickel (similar to iron) is one of the most common elements.
Just like iron nickel prefers to react with sulfur instead of oxygen. The resulting compounds are of mid level hardness (around Mohs 5).
While on earth a large part of the nickel is inaccessibly locked up in earth's core,
metallic asteroids (which are essentially broken up proto-planetary cores) allow direct access to all this nickel.
There nickel is the second most common metal right after iron. Concentrations of nickel in metallic asteroids can go up beyond two digit percents values.
Due to it's abundance nickel will thus likely find vast application in space technology.
Stochiometry & Diffusion
There seem to be very view nickel minerals with a clear stoichiometry. This may speak for lots of vacancies that provide pathways for diffusion Mechanosynthesized structures (which are practically faultless) though should be sufficient for suppression of such possible diffusion pathways. One nickel compound which as a clear stoichiometry even as natural minearl is bunsenite (NiO).
- Bunsenite NiO  (Mohs hardness 5.5)
(TODO: Investigate further to find the best nickel based abundant element gemstones.)