It appears when ever they wanted. Its true crystallization only occurs under intense heat, with the present known material science limitations. The bonds of material can be broken and reformed as often as you want when you understand the underlying science, which I don't. Heat is just the first way you discover that those bonds can be broken and reformed. And you learn that the cooling rate determines the crystal size. The slower the cooling rate, the larger the crystals. But you could argue for the existence of a hypothetical cold bond breaking 'tech' by observing the crystal size in that inscription.
In TLSOS, they depict that inscription as using the rock as a quick note pad to communicate something. The haphazard rough script clarity supports that conclusion reasonably well. But considering the clearly visible crystal size, if heat was a critical part of this 'tech' then they would have to have encased this stone in a device that slowly decreased the heat of the rock having been heated by the tool, over a long period of time to allow for that size of crystal. But as it seems to have been a quick scribble, I would conclude that its unlikely they had that problem.
Edited 1 time(s). Last edit at 07-Jan-20 15:06 by Open mind.