Crystals form due to good durations of 'good conditions' for crystal formation.
If for example it cools of quickly very small or no crystals may form.
Namely: Time, space and heat. Well, something to make the crystals out of, to. ("Building materials")
It needs time to grow, space to grow in and heat to keep the "building materials" flowing.
I suppose you could let it
re-crystallize by replicating/copying the environment in which it was created in the first place.
Then allow it to "grow" there a while. I say re-crystallize since I think it needs to be partly or wholly dissolved before being crystallized more. (either that, or you use it as a "seed" upon which the other crystals shall grow)
Have you heard of hydrothermally grown quartz?
It is a synthetic quartz grown in (sealed) high pressure water cookers, I suppose you could say.
Also known as autoclaves, or bombs.

Exactly the same thing; replicating the natural conditions in which quartz grows to form crystals.