GURPS Magic - Creating gemstones

[Calliban]

In GURPS Magic, Earth to Stone explicitly say it can’t create gemstones, but allow for creating basically any “simple” metal like bronze (actually an alloy) or iron. There are long discussions on this board about the meaning of “simple metals” (is gold valid? What about mercury? And steel?), but I am not interested this topic

What I am looking for is the reasoning for why gems were excluded, and what would a spell to create them be like - is there even an official one?