I wanted to say something similar but wondered… COULD you actually do a corundum-to-blue-sapphire or quartz-to-amethyst gradient, by growing the gem in a lab & gradually adding the doping (“dirty”) element?

I see your edit & yeah this isn’t melting, but does OP really care about the specific process!?

and a lab-grown gem would still be…clear & sparkly like a gemstone. the resin suggestions kill the whole thing imo

edit: similar to u/highfructosetrashjuice’s comment in the thread.