Cerium
Cerium is not the most abundant element. But also not extremely rare.
Cerium is possibly of interest for early primitive force applying mechanosynthesis
as CeO2 has a particularly high lattice scaled stiffness.
The lattice scaled stiffness of pyrite FeS2 (pyrite structure) is still a bit higher.
CeO2 is cubic, space group: Fm3m, #225
Natural occuring as the cerianite
solid solution mixing with Thorium substituting for cerium.
Since ThO2 has the same structure it seems likely that
the whole range of checkerboard compounds should be stable and accessible.
Natural thorium is rather weakly radioactive (like natural uranium)
thus safely as a solid non dusting mineral with a bit of care.