sorry for this opinion if you find it absurd. But this is a silicate type mineral if I'm not mistaken. either calcium, magnesium or a combination of sort. the structure may function as a tooth dentin. well, this is how we treat cavities, and some cracks in dentin...
If you don't mind the scarring that it will leave on it, have a dentist use the closest shade composite possible to bond it with, if returns are not possible, and if you are stuck with it. He will use a small diameter medium grit diamond bur to create a trough 1mm isthmus (width) and 1mm deep along the crack (trench). Then he will proceed with micro sandblasting the surface gently, and minimally to expose more rough silica to allow good bonding against. He will then use 35% Phosphoric acid to etch and to lift all debris the diamond left behind exposing the microscopic silicate surface, rinse well. after this, he will apply dental bonding agent for 20 seconds and cure this with the special light. now comes the filling stage. a closely matched Dentin shade (opaque) composite filling packed inside this groove, shaped, will be cured. Then polish to a shine.
It will hold. It will not be perfect match since composites are build to match teeth (made to have translucency), but it will work for function. And if dentist fellow is creative with the drill (handpiece), he may turn this occasion into a design, or pattern.
I would do it for you if you were local, and time was not an issue. But contact a local dentist and talk to him in person. He may be pleased that you went to inquire of his special skills for this project.