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 second 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] - hongquiite - 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 third 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

Given silicon is a semi-metal and titanium is a metal titanium silicides should come with quite metallic properties (optically and electrically).
But mechanically still gemstone like like inter-metallic compounds.

Zirconium Zr compounds (maybe)

• Zr (fifth row) is the element below Titanium (fourth row)
• Zr is the most abundant fifth and below row non alkali element (Earth's crust).
• Zr makes similarly good compounds with various other elements as Ti

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 – (germanium Ge is rather rare)
• Other: MnO₂, FeSbO₄ – (antimony Sb is rather rare)

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 - ok if well sealed inside of products

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 bunsenite - (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
• Simple crystal structures of especial interest
• Can we make these high potential base materials from random stones lying around on the ground? Yes! See: Common stones
• Abundant element