So I created my account like 1100 days ago. but started playing now.
So from what ive understood is that main ways to get gems are Mining, Nether, or Farming. But I dont know what farmable is the best.
And if I were to get gems what should i spend them on for max profit?
I recommend you try fishing buy some rod and fishing you will profit if u get much 
i think spike bombs are one of the best. Fishing is great way to get gems too and pets too.
which rod would be the best and do I just turn the fish into gems or should I sell them in PWE?
Titanium superior is best but its very expensive.
which rod would be the best and do I just turn the fish into gems or should I sell them in PWE?
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Do not do that. Instead (depending on your budget) buy a pro fisher set, whistle, glasses, Bamboo superior or carbon superior, and fish using a specific lure (common for superior, uncommon for carbon) . Once done drop all the fish in a world use the radio to sell the fish gems 2.5k/1wl.
You should definitely try Mining or Fishing. 
If you’re willing to sit through them I’ve had success watching ads dierctly from the store and from the tv’s getting 1000 gems in total
i can get many gems by nether.
fishing is a little bit hard for me since i still have the basic rod.
mining is cool, but the mobs suc-
Good luck, and oh yea. dont waste your gems and use it wisely.
Mining is the best way to obtain gems, don’t try fishing i don’t recommend you it unless if you are willing to afk all the time.