What do you do with unused lvl 2+ gems?

Sometimes we equip a gem for a while and it gains a few levels. Then we realize that there is no room for that gem in the perfect 9001k dps Ubermensch of a char that we are creating. So we put it in inventory until we figure out what to do with it, much like how folks in their forties put tools and sports items in their basement/garage.

My best thought is to take an unused leveled gem and bring it along while farming, so it hopefully gets to lvl20 and then I can use the glasscutter''s bauble recipe to create a +20% lvl1 gem.

I think it depends on the gem.

I believe there are still a few that if they drop are considered worthwhile to keep but in general the best policy is that unless they have some quality on them you leave them on the ground or sell them to the vender otherwise you'll end up with stash tabs full of useless gems doing nothing but taking up space.

Of course you can put some gems in some spare slots and level them up either in case you want to use them again or to make them 20% +1 but there are only so many slots you'll have spare to do that with.

There are also some gems which are more valuable than others when 20%+1 or 20%+20 which means that it could be better to pick a different gem to farm with anyway rather than the one you've currently got partially levelled.

Can you buy it from a vendor? If so:
Is it superior but under 10%-ish quality? GCP recipe.
Is it below the level for gems you can buy from a vendor? Scroll fragments.

Otherwise, maybe keep it, or put it in my 1c sale tab.