The moment it becomes useful its cost will fall down to silicone prices really fast.

Right now you can buy 100g of diamonds for low teens in $ from China. Diamonds are produced in enormous quantities as abrasives. And if you do something on large scale you become better at it.

So far we haven't invested in big monocrystals because there was not much use for them aside from jewelry and probably other small niches. The moment some R&D makes something useful out of them - they will be Jevoned into oblivion.