As of now, only Crystallized Zinc is available to find in chest, out of the 25 available.
Is this voluntary?

The reason why that's the only one found in chests is that unless there's a mod present that adds in Zinc (or Brass, since Brass is a copper-zinc alloy), it's pretty much impossible to obtain Zinc in a crystallized form. Might eventually have more crystals showing up as loot when there's more use to them (the Materializer is getting closer to being fully implemented, among things).

Ah ok, didn't understand since I always use Metallurgy or Base/Modern Metal lol
No use keeping that open, as it's Intended Behaviour