In most games, where active skills are involved, the process is usually simple. You just pick the skill and play with it all the time without any possibilities to alter how it works and feels, which is kinda boring. You won’t have to worry about that in Path of Exile. In PoE skills work in a different way – every skill is also an item which can be altered with help of other skills, unique items and passives from the tree. Skills drop from monsters as Skill Gems. Here is an image to show, how skill gems look when they drop
There are 2 big groups skill gems are divided into – active skill gems and support gems.
Active skill gems are used as attacks mostly and support gems are used to alter active skills they are combined with in some way. For example, active skill gem Ice spear shoots one spear out in the direction it is used. When combined with support gem greater multiple projectiles, you shoot out 4 extra projectiles in cone. You can see how it looks after adding support gem in this image.
There are quite a bunch support gems in the game, for example you can make skills attack, cast faster, increase their area of effect, spend hp instead of mana when casting them and many more options.
Skill gems come into 3 colors – blue, green and red. Each color requires different attribute to use and level skill up. Blue ones need intelligence, red ones – strength and green ones dexterity.
To use gem, player has to put the skill gem into one of the colored sockets in his gear. They can only be put into socket with corresponding color – you cannot put green skill gem into blue socket of the item.
If you want to add support gem to your active skill gem, you need item with at least 1 link. 1 link means 2 sockets of the item are connected to each other. If there are no links, gems will not work together.
For example, if I want to make my Ice spear work together with Greater multiple projectiles, I need item with 1 blue and 1 green socket and they have to be linked together.
Usually gems aren’t worth a lot, although there are few exceptions. Most of them can be obtained as quest rewards, so I advise you to collect gems with quality (you can sell them or use for Gemcutter’s Prism recipe) and gems that aren’t obtainable as quest rewards.
Most expensive gems are – Empower, Enhance, Portal, Enlighten. There are some widely used gems you could pick up, such as Multistrike, Spell Echo, Increased Area Of Effect, Reduced mana, Reduced duration etc. You will pick up on what’s useful and sellable by playing the game yourself mostly.
You can buy Path of Exile gems from other players or by visiting vendors, both ways are easy.