Pure metals and metal alloys

These may often not be suitable for gemstone metamaterial technology due to

• surface diffusion at room temperature and
• limits on passivatability.

Actually it depends.
With no grain boundaries and no internal vacancies or interstitial defects
many metals will likely feature no internal diffusion at room temperature.

If surfaces are completely flat all the way to full crystallographic plane turns
with no crystallographic steps on the planes, then there usually won't be any diffusion on the surfaces too.

Accumulated defects from radiation may start to diffuse around though.
Also mechanical overload may lead to introduction of defects that start to diffuse
instead of staying pinned down or causing a "clean" break right away,like in the case of the more covalently bonded typical gemstones.