Carbon

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See: Gemstone like compound#Carbons versatility

sp3 allotropes:

sp2 allotropes:

  • Graphite (today single cystalline graphite is called HOPG for highly ordered pyrolytic graphite)
    Future: POPMG for "perfectly ordered piezomechanosynthesized graphite.
  • Buckyballs (convex curvature; concave from inside) – weak molecular solids
  • Nanotubes (no curvature – flat rolled)
  • 3D meshes (hyperbolic curvature)
  • "penta graphene" (cairo pattern)
  • ...

No exactly defined structure:

Exotic allotropes:

  • ...

Binary compounds:

  • Silicon carbide SiC aka moissanite
  • Carbon nitrides: beta carbon nitride and cubic gauche carbon nitride
  • Titanium carbide TiC aka titanium-khamrabaevite (cubic rock salt structure) Mohs 9-9.5 refractory
  • Vanadium carbide VC (vanadium is not too common) vanadium-khamrabaevite
    similar is: NbC and TaC (Nb is not abundant, Ta is extremely rare)
  • Zirconium cabide ZrC [1] (same structure but no natural mineral present)
    similar is: HfC (Hf is pretty rare)
  • Iron carbide (this here is not cementite!!) iron-khamrabaevite (unknown stability, likely very hard)
  • Chromium_carbide (various stoichiometric & structures - may point to useful covalent behavior)
    Cr3C2 Tongbaite (refractory, Mohs 9.6; orthorhombic; 6.64g/ccm) Cr is not too abundant
    Cr23C6
  • Molybdenium carbide (de) Mo2C (insoluble, two modifications α and β) Mo is rather rare
  • Tungsten_carbide (hexagonal, Mohs 9) W is rather rare
  • Fe3C, Ni3C, Co3C cohenite endmembers (likely rather metallic, Mohs 5.5-6)
  • copper and zinc are more electronegative => more covalent behavior => organometallic compounds
  • Boron carbide B4C [2] (boron is not too common)
  • Aluminium carbide [3] (reacts with water - releases methane gas CH4)
  • Beryllium carbide Be2C (very hard but reactive, toxic and rare)
  • Magnesium carbide ??? - magnesium acetylide Mg2C3 (de wikipedia)
  • Calcium carbide CaC2 (an acetylide - reacts with water - releases ethyne gas C2H2)
  • TODO: La, Ce, (Li, Na, K)

Way harder than diamond

Macroscopic (pre)tensions can increase resilience but not ultimate tensile strength.
In glass there internal tensions can increase resilience massively.
See Wikipedia on: Prince Rupert's drops
These drops have their tail as weak spot and their shape is pre-given so they are of not much use beside a curiosity for sci-education.
In principle using advanced bottom up manufacturing it might be possible to toughen diamond a lot via internal tensions but leaving no weak spot.

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