Macroscale surface passivation
Almost all metals except the most noble ones form more or less
stable protective oxide layers on their surfaces when
exposed to oxygen and eventually also moisture possibly with salts dissolved.
There are very few exceptions including e.g. Gold.
Even the quite noble metals copper and silver quickly form passivation layers on their surfaces
when observed at the nanoscale.
When we touch metals what we touch are actually the protective oxide layers on their surfaces.
Not actually the metals themselves.
What gemstone you actually touch when you touch a metal you can look up on this page here:
Passivation layer mineral.
The only other thing than a passivation layer mineral that you may unknowingly touch when touching what seems like a metal is a layer of clear (organic) lacquer.
More nobles metals like copper and silver react with other things present in the atmosphere than oxygen to a good part.
When the oxide layers are intentionally artificially thickened (for optical and or protective effect)
e.g. by anodizing (but there are other means too) then
the passivation layer mineral is easily optically visibly, sometimes in bright iridescent colors due to interference effects of the light with the thin
transparent oxide layer.
The naturally forming protective oxide layers are often thin enough to be optically transparent.
Especially when still young on very freshly created surfaces (by breaking, casting or deforming).
But the first atomic layers of oxides usually build up extremely fast.
This would be devastating for exposed nanomachinery made from metal.
So there won't be such a thing.
Relation to nanoscale passivation
Nanoscale parts made from metals or alloys rather than made from already oxidized gemstone
would in most cases quickly oxidize when exposed to air forming a thick imperfect oxide layer that is thicker than
the whole part itself and thus destroying the part.
Most nanomachinery will be well encapsulated and located inside of
macroscopic gem-gum products and thus well protected from things like oxygen.
