Is (high) pressure required to form (quartz) crystals?
Pressure does not affect the crystal structure (unless it gets insanely great, so that stishovite formation kicks in; that's not going to happen in an autoclave, though). Pressure, however, does affect the temperature (indirectly), and hence solubility, and hence the rate of crystal formation. There is no lower threshold, that's why you keep getting wildly different estimates for the "required" pressure. It is just that the higher you get, the faster your crystals grow. They would grow at normal atmospheric pressure as well, just very slow (so that only your great-grandchildren may possibly be able to see them).

I suggest that you aim at something less extreme. Calcite, maybe?