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

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

Second row elements:

• TiP - phosphid Titan(III) phosphide (de.zxc.wiki) - hexagonal - 1860°C - 3.94g/ccm - optically metallic
• Ti₃P (materialsproject.org) - 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)
• Ti₅Si₄ - 2120°C - tetragonal (isotype to Zr₅Si₄)
• TiSi Titaniummonosilicide - 1760 °C - orthorhombic (isotype to FeB)
• Ti₅Si₉ - spacegroup Cmcm (Nr. 63) - 3.9g/ccm
• Ti₅Si₃

Quite simple rutile structure & Hard

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

Neo-polymorphs with rutile structure
See: the stishovite continuum currently on the page about silicon
Silicon group: GeO₂, SnO₂, β-PbO₂
Other: MnO₂, FeSbO₄

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

• 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 beryllium is scarce and poisonous