Pure metals and metal alloys

Pure metals and (checkerboard pattern mechanosynthesized) metal alloys
may often not be suitable for gemstone metamaterial technology.

That is 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.

Some metals have quite covalent character:

• Tungsten (it's very rare though so not very interestig as a structural material)
• Tin in its covalent "grey tin" phase where it crystallizes into a sparsely filled high volume crystal structure like silicon and diamond (elements of the same group)
  Mechanical properties of solid (mechanosyntesized) grey tin are as of yet unknown.
  We only know it as a powder originating from an usually undesired phase transition.
• Copper needs only to be put into a 2:1 ratio with oxygen (Cu₂O) and it already becomes a transparent nonmetallic gemstone called cuprite. Quite odd.
  It seems it's nobleness (few available hull electrons to give away to form bonds) puts it near to nonmetallicness.
  It's not mechanically brittle on it's own though (like tungsten and alpha tin).
  Quite the opposite actually.