Biodegradable gemstone

This will be taken a bit more generally that just lifeforms using it as food.
That is this will (also and more prominently) including processes
that erode the material away in reasonable timescales
and the effluents released slowly not being toxic to life but rather best case acting as fertilizer.

Simple cubic rock salt structure

Most of these will be too brittle and weak from many structural applications.
Single crystals should not flow though as large mountain scale agglomerations do.

★ alkali metal halogenides aka classical salts like NaCl NaBr KCl KBr NaI KI …
These are usually too water soluble, could be used in perfecty sealed interiors though.
This then shifts the problem of biodegradability to the sealing shell.
defeats the purpouse of biodegradability as the sealing shell

★ earths alkali chalkogenides like MgO CaO MgS CaS
– Some are quite promising like MgO with decent hardness and decently low water solubility
– Some are bad like CaO wich not only dissolves but energetically reacts with water. Needs sealing like salts.
– The sulfides are quite soft but occur as minerals speaking for their lower (but nonzero) water solubility

★ transition metal monoxides (and nonmetallides)
– Particularly with abundant element titanium: TiO TiN and TiC
– More abundant iron FeO wüstite works but not a high performance gem

Oxidic gems

Generally the softer the gem the better degradable (by tumbling and abrasion).
★ SiO2 (low): in quartz form: Mechanically degrades slowly as can nicely be seen with seaglass though this is often accelerated due to mixed in K or Na
★ Al2O3 (no): in sapphire/corundum form: Very hard very slow virtually no degradation
★ TiO2 (low to mid): in rutile/anatase/brookite form: Mohs (5.5-6.5) softer than quartz (Mohs 7) thus likely faster degrading
★ Salts of oxoacids (mid to high): These are typically softer gems even for the harder ones like appatite(Mohs 5)

Biominerals

★ Most of these are salts of oxoacids.
– Most prominently CaCO₃
=> calcite (trigonal, Mohs 3 defining mineral)
=> aragonite (orthorhombic, Mohs 3.5-4.0)
… very apparently much remains inert and unrecycled by life to the point for forming mountain-ranges
★ Non oxoacid exceptions include:
– fluorite CaF₂ (cubic, Mohs 4 defining mineral)
– pyrite FeS₂ (cubic, Mohs 6.0–6.5)
– magnetite Fe₃O₄(cubic, Mohs 5.5–6.5)
– goethite α-FeO(OH) (orthorhombic, Mohs 5.0–5.5)
– brucite Mg(OH)₂ (trigonal, Mohs 2.5–3.0)
– dolomite CaMg(CO₃)₂ (trigonal, Mohs 3.5–4.0)
– atacamite Cu₂Cl(OH)₃ (orthorhombic, Mohs 3.0–3.5) - note the copper
– struvite (contains Mg,P,N,H2O) (orthorhombic, Mohs 1.5–2)

Spill of gems in nanoparticulate form

If the gem-gum product is skin sealed and the seal of that skin is broken open some may oxisdize right away.
See: Oxidation#Fruit interior analogy
The natural passivaing oydatin layer thickness that forms in seconds
being way thicker then the size of the nanoscale parts (crystolecules).

External links

Wikipedia:

• Biodegradation
• [1] aka Niningerite(Mohs 3.5 to 4.0)
• Calcium sulfide aka Oldhamite (Mohs 4)
• Magnesium_oxide magnesium oxide aka Periclase (Mohs 6)
• Calcium oxide