With defiant gear becoming so prevalent and the lack of good sellable drops in SoD means that gem vendor trash might be making a comeback. I've notice that tradeskill crap doesnt sell worth anything anymore either. Rank 2 spells sold well for a while but at this point, just about everyone has them. CT revamp will probably mean that just about all armor will be worthless, will be interesting to see what that gear sells for.
Some notes from WAY back in the day when gems were one of the best ways to go:
Just about all PoP zones have good diamond rates. PoFire is really good since the mobs are close together so you dont have to wander nearly as far as other places. Bad news is that it is good xp for lower levels so the castles might be camped. Sol Ro Tower isnt horrible but mobs are REALLY spread out.
Seb is good for mass slaughter and decent gems plus nodding blue lilies which might sell well still. Can make potions out of them, might sell well still. Pull everything, slaughter everything, decent plat at 85 depending on how fast you can clear the zone. Same goes for Dulak. Bonus for Dulak is you can make illusion pots from the meats and parts that drop. They normally sell alright.
Also, I used to slaughter PoHate and PoFear for plat. Dont know if there is still the quests active but the crappy class specific armor that drops quite commonly from trash can be turned in for a gem i believe. It is another option for you and can be quite fun to break into fear alone if you remember the horror if raiding PoFear, oh about 8 years ago or so
Another option is to slaughter Vald while it is a hotzone. The spells that drop can be vendored for like 500pp a pop or more. Depending on how fast you can kill, this might be worth it. Ashengate or DW cliffs might be similar.
Hill Giants are always an option, but might be more annoying than worth it. Would think they are still pretty much perma camped.
These are NOT get rich quick schemes. Whatever you choose, you will not earn plat at an amazing rate like you used to be able to farming bazaar crap. However, you should make steady plat. If anyone knows better ways to get plat, I'd love to hear it
"Sometimes you have to face your demons, sometimes the demon is you." - Yesak Demunstorm