Oxides

(TODO: discuss major ones here)

Carbon group / group IV / group 14 oxides

CO₂ normally the well known gas but highly explosive when in solid sp3 form (analog to quartz)
SiO₂ quartz and its various known polymorphs (~Mohs 7)
GeO₂ argutite Mohs 6-7 -- germanium is a rather rare element!
SnO₂ cassierite Mohs 6-7
PbO₂ [1] -- alpha-PbO₂ scrutinyite -- beta-PbO₂ plattnerite (TODO: find out Mohs hardness of PbO₂ minerals)

(All those have rutile structure. See page about silicon.)
(TODO: Here maybe some pseudo phase diagrams can be made.)