Base materials with high potential

Very good materials

Best of the best

• diamond and its polymorphs including hexagonal diamond aka lonsdaleite

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• moissanite - SiC - high heat resistance
• stishovite and seifertite - metastable ultrahard and dense SiO₂ polymorphs
• simple titanium gemstones including: TiN TiP TiC TiSi₂ TiB₂ TiO₂ Ti₂O₃

Simple rutile structure & hard

• rutile TiO₂
• stishovite - metastable SiO₂ polymorph - rutile structure & very hard and dense
• neo-polymorphs with rutile structure

Simple salt structures

• periclase MgO
• some transition metal monoxides (TiO, FeO, NiO, ..)

Other

• spinell MgAl₂O₄ - very hard and cubic

Materials with some weakness

Con: low crystal structure symmetry

• leukosapphire - very hard
• quartz - and other low density polymorphs of SiO₂

Con: Rather soft materials

• calcite and aragonite

Others

• garnets - hard and cubic (but big unit cell)
• Brommelite BeO - very hard but scarce and poisonous