Apatite
A calciumphosphate
- Ca5(PO4)3(OH)
- Mohs 5 (defining mineral)
- hexagonal
There are hydroxy, fluorine, chlorine, and even bromine versions.
Fluorine has more influence on chemical resistance than mechanical properties.
In advanced gem-gum nanosystems the interior will be well sealed and fluorine is not to common and can be toxic when released. So in most cases the hydroxy (or chlorine) version will likely be preferable.
Apatite is a well known biomineral forming our teeth and bones.
Side-note: Ceramics can and has been made. It's called "bone china".
But the interest here is on single crystals for crystolecules.
initiated by addition of turquoise which is harder than apatite the best known phosphate
(which is making up our teeth and bones and is forming green gems)
Phosphates harder than apatite
There are harder phosphates than apatite possibly at the expense of lower symmetry crystal structure One example being Turquoise
(wiki-TODO: continue skimming for the hardest phosphate with other properties being good too)
