When casting metamagic - fork spell and applying it to Create Jewelry: it produces two rings from only one gemstone; but when you have a stack of more than one the same gem it will consume two gems to make two rings.
With a full inventory it seems to run into the same bug that is destroying items when opening chests, one ring takes the spot of the gem but the other disappears. This is probably the same fix as noted in the other bug report, but might be worth verifying.
Fork magic in general might need balancing with regards to spells that consume resources other than mana, although this seemed the most egregious example. I didn't test with the high level black magic spell that uses an onyx, and forgot to test it with the earth magic that creates mana crystals from gems. Both of those also seem potentially problematic.