Late game you'll have to get into gem slotting to upgrade your gear into having more sustain or more damage.

Gems are a must for evolving in Archlight, each helmet, armor, legs, boots and weapon can have up to 7 slots for gems. Those slots can be opened by using Regrade Keys.
Gems are made by Jewelcrafting, and each gem have 2 possible effects.

Gem Possible Outcome
Forest Gem Green3.gif Attack Spell Cooldown or Attack Speed
Inferno Gem Red3.gif Physical Power or Spell Power
Cloud Gem Purple3.gif Resistance or Rune Damage
Ocean Gem Blue3.gif Mana Regeneration or Max Mana
Mountain Gem Yellow3.gif Health Regeneration or Max Health
Order Gem Order gem.gif Loot Find or Essence find

Gem Values
Those gems can be removed all at once from gear by using a Gem remover. Item 21804.png

Jewelcrafting currently offers 3 tiers of gems. Normal Gems, Charged Gems and Overcharged gems, each gem is stronger (min-max rolls) than the previous one. Below you will find possible builds (templates to direct players on how they can build. Your build can be different, its all up to personal preference.)

The tankiest way:
There are two debatable ways to achieve this: Mountain/Ocean Gems or Cloud Gems.
Considering that everyone needs some form of sustain late game, every % based recovering items will have a huge impact on your self healing (Refreshing set and life leech/clear state enchant).
On the other hand, getting damage reduction gems will help mitigating received damage and not needing much self healing.

The damage dealing way:
Either Forest or Inferno Gems are the way to go for this!

You can have as many gems of the same type as you want, but after 11 rolls of the same effect you will receive something called diminishing returns, these were made to prevent people from building up full damage and/or full resistance and either killing people instantly or becoming unkillable. You will receive the following penalties for each extra gem effect:
But hey, what do you mean exactly by penalty?
Answer: Once you hit 12 gems with the same effect, all of your gems on the last piece of gear will receive a -25% efficiency.

Gems Penalty
12 25% for each further one
20 35% for each further one
25 45% for each further one
30+ 55% for each further one
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