Everybody (who got this far) knows that Limmir, the jeweler of the Valley of the Moon quest, will add a random third gem to the two that you give him. If you have the "See the Threads" skill, e.g. from the Threads of Fate cloak, then start the skill just before you give Limmir your precious Goedelath Rock, have three rolls of those random gem dice, and choose the best one.
I can think of using the same technique with Wyrm Bile but it needs testing
Nice idea! Will test and report back (unless somebody else beats me to it). Just as an aside, in case people start wondering: this does not
work for the merchant artifacts. The spell will fizzle after the first timeline completes, and you will be left with the artifact you got then.