In 2 Cravan-Liquids-worth (~20 minutes, since they last 10 minutes each) of metallurgy in the poison swamps, I think it's safe to say at least two things:
1) The gem find rate is noticeably higher than metallurgy in normal spots.
2) The gems you do find are still up to RNG to decide what they are and how small/large they are.
First gem I found was a 1.01cm Jasper.
Most recent was a 4.86cm Diamond.
Also you can find both beach and river version of old gems at the swamps, not that that's all that great, considering their comparatively low value.
I hadn't found any of the new gems, and the alligators are really annoying to deal with, and even Divine Link won't help THAT much, since just because YOU drink the Cravan Liquid to gain swamp immunity doesn't mean your pet will be as well.
I guess if you wanted to feed one to your pet as well as yourself... it might be worth it to lower the hassle of "pet out of alligator aggro range while you, gathering, are within range*.
Hey, on the upside, the poison swamps are excellent for training your Divine Link's Sacred Revival subskill.