Skill gems include active skill gems and support skill gems. First of all, you need to equip a skill gem into your item socket. Then the skill gem will grant you a certain ability. Your every skill come from gems.
Skill gems have levels and quality and it can increase the effect of the gem. There are many vendor recipes for skill gems.
1x Skill Gem’s level -1 = 1x Skill Gem + 1x Orb of Scouring
Sell any skill gem with a level plus an Orb of Scouring to any vendor, the skill gem’s level will minus 1. For example, a level of 15 skill gem plus an Orb of Scouring will exchange for a skill gem of level 14.
Since Vaal Gems cannot be modified anymore, this vendor recipe cannot apply for Corrupted gems.
1x level 1 skill gem = 1x skill gem + 1x Orb of Regret
Plus an Orb of Regret to a level skill gem recipe, the level will reset to 1. Be careful, any level will be reset to level 1. Orb of Regret is a PoE currency can also grant a passive skill refund point for a skill tree.
This recipe cannot apply for Corrupted skill gems as well.
1x 20% quality skill gem = 1x level 20 skill gem + 1x Gemcutter’s Gem
Gemcutter’s Gem is an expensive PoE currency and improves the quality of a gem. To reach 20% quality, you need 20 Gemcutter’s Gem. This method is useful to save a lot of Gemcutter’s Gem. The return skill gem is the same type of as the old one.
Vaal skill gems cannot use this vendor recipe.
1x Block Chance Reduction Support = 1x Puncture + 1x any dexterity shield with 20% quality
Block Chance Reduction supports any skill that hits enemies. This support gem can be only obtained by the vendor recipe. A Puncture skill gem plus any 20% quality shield with dexterity property can vendor for Block Chance Reduction gem.
Blink Arrow is an attack that fires an arrow, upon reaching the destination you are teleported there and a clone is summoned at your previous location. Mirror Arrow is an attack that fires an arrow, dealing damage and summoning a clone when it reaches its destination. The clone is a minion that uses the player’s bow and quiver.
You can purchase these two skill gem from Siosa(The Library, Act 3) or Lilly Roth(Lioneye’s Watch, Act 6).
Bitterdream is a unique shadow Sceptre and 22% increased Elemental Damage. Sell a Bitterdream to any vendor, you will get the following gems:
Selling The Hungry Loop to a vendor returns any consumed support gems.
The Hungry Loop is a unique Unset Ring. It consumes socketed support gems when they reach maximum level. When a support gem is consumed, the Hungry Loop gains a modifier that supports socketed skill gems with the maximum level of the consumed support. Gem quality is ignored. The Hungry Loop can consume up to four different support gems.
There are several methods of obtaining skills and support gems:
(1) Earned through a quest reward
(2) Purchased from town vendors once a certain quest is complete
(3) Vendors in Acts 1-4 sell leveled skill gems. The available gems are based on the character’s class.
(4) Siosa in Act 3 sells all unleveled skill gems that can be received as quest rewards, across all classes.
(5) Lilly Roth in Act 6 sells all unleveled skill gems after completing the Act 6 quest "Fallen from Grace".
(6) Vendor recipes.
(7) Random drop from monsters, chests and strongboxes.
(8) Random offer from a master.
(9) Vaal skill gems come from Vaal Vessels in corrupted areas.
(10) As a reward for turning certain sets of divination cards.