Stable carbon storage gemstones

There is a particular focus on using highly abundant elements
to tie the carbon down nigh permanently
preventing it to ever get back into the atmosphere.

Excellent highly stable carbides

These are slack forming and fire self quenching.
These are very hard and resistant against erosion and bio-degradation.

Gem grade silicon carbide SiC aka moissanite

• Silicon as element is most abundant right after oxygen and highly accessible right after nitrogen
• SiC is extremely thermally stable, much more so than diamond.
• SiC can't be ignited in air, neither by natural means nor artificially
• SiC does not naturally erode or biodegrade (which can be seen as a problem too)
• SiC is quite chemically stable, but it can be dissolved as everything can given aggressive enough unnatural(!) liquid chemistry.

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Titanium carbide TiC aka khamrabaevite

• Titanium is most abundant transition metal element after iron which makes much less stable carbides.
• TiC is extremely thermally
• TiC likely does not naturally erode or biodegrade (which can be seen as a problem too)
• TiC is quite chemically stable. Since it is electrically conductive electrochemical degradation is likely possible

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https://en.wikipedia.org/wiki/Titanium_carbide

Boron carbides

• Boron is less abundant and accessible as element
• Boron carbides are extremely thermally
• Boron carbides likely does not naturally erode or biodegrade (which can be seen as a problem too)
• Chemical stability likely high?

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• https://en.wikipedia.org/wiki/Boron_carbide B₄C – only one fifth of atoms is actually carbon here
• https://en.wikipedia.org/wiki/Boron_carbides – similar fro others

Diamond normal cubic form and exotic hexagonal form aka lonsdaleite

• more limited heat resistance than SiC
• can be ignited in extreme enough conditions (as metals can) or when in filligree form
• does not naturally erode or biodegrade (which can be seen as a problem too)
• is quite chemically stable, but it can be dissolved as everything can given aggressive enough unnatural(!) liquid chemistry.

Monocrystalline graphite

• similar to diamond
• slightly more heat resistance than diamond
• maybe slightly less fire resistant than diamond
• possibly less stable against erosion than diamond (flakes coming off)

Carbonic acid salts

• CaCO₃ calcite aragonite and other forms
• MgCO₃ magnesite and other forms

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• Both are slack forming and already oxided so completely fire immune.
• Due to being softer and slightly water soluble these do naturally erode (& biodegrade?) (which can be seen as an advantage in some cases)

Both Ca & Mg can be sourced from metal rich ultramafic silicate rock.
APM/APT may help making that energy efficient enough.
This is actually attempted by current day (2025) technologies
pumping CO2 in ultramafic rock to be permanently chemically bound
or bringing out crushed ultramafic rock on fields.

Less stable carbides

Iron carbide Fe₃C aka cementite or iron-cohenite

• Iron is by far the most common transition metal element thus self suggesting to use.
• Cementite is known from iron processing where small crystallites can inprove quuality of carbon steels.
• Mid erosion: The material as a single crystal on it's own has a decent mid hardness of Mohs 5.5 to 6.
  Much less than SiC and TiC
• Unclear biodegradability.

• https://en.wikipedia.org/wiki/Cohenite
• https://en.wikipedia.org/wiki/Cementite

Aluminum carbide (Al₄C₃) – bad choice

• Aluminium is the most abundant metal and nigh as abundant as silicon thus self suggesting

• Big issue: The "gemstone" readily reacts with water to aluminum hydroxide and methane
  Methane is a really bad greenhouse gass and dissolved aluminum hydroxide makes for aggressive lye
  that even neutralized as salt may pose health hazards.
• Quite thermally stable (1400°C in air, 2100°C melting)
• https://en.wikipedia.org/wiki/Aluminium_carbide

Related

• Carbon sequestration
• Gemstone
• Base materials with high potential