Quartz crystals are pretty common, the second most common mineral in the earth's crust, and so fairly cheap. They are not easily dissolved in water or any other liquid, do not easily melt or react chemically, and are one of the hardest common materials. The crystals have to be pretty stable or there wouldn't be so much of it found in the crust, it would all be reacted or dissolved away.
They can be synthesized - and while the exact temperature is difficult to find, that a common pressure cooker is used as a suggestion for making small crystals at home suggests that it is possible within your temperature range. However, quartz is common enough you may not need to synthesize them to have plenty of crystals for everyone. (one source claims the temperature is 345-380*C, so well under your range... though it does require 1000-1500 bars of pressure as well)
Monocrystalline, hmm - pure quartz is SiO2, an oxidated sillicon crystal that should have a pretty regular lattice (forgive if that wasn't what you mean). There are a lot of varieties, and some are more likely to be polycrystalline than others, but at least clear quartz, and probably a few of the more translucent, evenly colored, or stable, or something varieties might be monocrystalline as well.
Quartz can be annealed, at 1140*C (analogous to firing pottery, I think) - I think that's still in your "lesser than 1500*C" range?
Of course, the differing varieties, colors, and properties may or may not have effects on their magic use - or people might assume they do. Lots of fun to play with!