Base materials with high potential

Very good materials

Best of the best

Best diamondoids

• diamond and its polymorphs including hexagonal diamond aka lonsdaleite
• moissanite - SiC - high heat resistance
• pure silicon (eventually)

Best SiO₂ polymorphs

Metastable ultrahard and dense SiO₂ polymorphs:

• stishovite (tetragonal rutile structure)
• seifertite (orthorhombic scrutinyite structure)

Simple titanium gemstones

Titanium compounds with first row elements

• TiB₂ Titanium diboride - hexagonal 2D layered - 3230°C - 4.52g/ccm - optically metallic - highly refractory
• TiC Titanium carbide - simple cubic - 3160°C (800°C in air) - 4.93g/ccm Mohs 9 to 9.5 - water insoluble (almost)
• TiN Titanium nitride - simple cubic - 2,947°C - 5.21 g/cm3 - optically metallic (golden) - "barrier metal" - water insoluble (almost)

Titanium oxides:

• TiO [1] - simple cubic - 1,750C° - 4.95g/ccm - optically metallic (golden)
• Ti₂O₃ [2] - tristarite - hexagonal corundum structure (like sapphire) - 2,130°C (decomposes) - 4.49g/ccm - semiconducting to metallic at 200°C
• TiO₂ [3] - rutile, anatase, brookite, and more

Titanium compounds with second row elements:

• TiP - phosphid Titan(III) phosphide (de.zxc.wiki) - hexagonal - 1860°C - 3.94g/ccm - optically metallic
• Ti₃P (materialsproject.org) - tetragonal - 4.7g/ccm
• TiSi₂ Titanium disilicide - orthorhombic (complex unit cell) - 1,470°C - 4.02g/ccm - water insoluble - optically metallic and electrically conductive - More titanium silicides ...
• Ti₃Si - tetragonal - (isotype to Ti₃P - see above and Zr₃P)
• Ti₅Si₄ - 2120°C - tetragonal (isotype to Zr₅Si₄)
• TiSi Titaniummonosilicide - 1760 °C - orthorhombic (isotype to FeB)
• Ti₅Si₉ - spacegroup Cmcm (Nr. 63) - 3.9g/ccm
• Ti₅Si₃

Zirconium Zr compounds (maybe)

• is the element below Titanium
• is the most abundant third row element (which are all rather rare)
• makes similarly good compounds

Quite simple rutile structure & Hard

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

And neo-polymorphs with rutile structure. These include:

• Silicon group: GeO₂, SnO₂, β-PbO₂
  
• Other: MnO₂, FeSbO₄

See: rutile structure. There is also a mention on that on the page about silicon
This could be called the the stishovite continuum or the rutile continuum'.

Mono metal monoxides (simple cublic NaCl salt structure)

Earth alkali based

• MgO periclase
• CaO - questionable - highly reactive with water

Transition metal based

Some transition metal monoxides (Typical: Max 1300-1900°C - Mohs 5-6)

• TiO hongquiite
• MnO manganosite - (Mn is less abundant)
• FeO wüstite
• NiO brommelite - (Ni is not too abundant on earth but very abundant on metallic asteroids)

V vanadium, Cr chromium, Co cobalt do that too but
these elements are more scarce thus
not included as pure high volume base materials here

Other

• spinell MgAl₂O₄ - very hard and cubic

Quite good materials with some hampering weakness(es)

Con: low crystal structure symmetry

• Al₂O₃ - leukosapphire - Mohs 9 (defining material) - (isotype to Ti₂O₃ tistarite)
• SiO₂ common 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 beryllium is scarce and poisonous

Related

• Gemstone like compound