Nearly all action RPGs had linear skill systems where the progression of an active skill was preordained by the developers. If you were a fire user, you would progress from a weak fire spout to a more powerful fireball, and eventually to something big and flashy like a flame wave.

In Path of Exile, active skills (i.e., skills that you use on command) are itemized onto "Skill Gems". These gems confer an active skill to the player when they are socketed into equipment and gain experience as the player slays enemies.

A choice of staple skill gems are given out as quest rewards throughout the game to ensure that players have a variety of skills available. You’ll discover rare advanced skill gems out in the wilderness and dungeons. Like all items, these can be traded between players. Some of the more devastating skills are extremely valuable.

Gems can also level up. When a player is rewarded experience, all gems in equipped items gain 10% of that value. This gain is in addition to the experience earned by the player. When a gem is removed or traded, it retains its experience. It’s possible to specialize in leveling up valuable gems and trading them to other players. We haven’t announced the number of levels that gems will have, but it takes a substantial amount of experience to reach the highest levels of gem potency.

Gems have attribute requirements based on their type and level. For example, a higher level cleave skill gem requires more strength than a low level one. If your character doesn’t meet the attribute requirements of the new level of the skill gem, then the experience value sits at the maximum of the previous level until you next gain additional experience and also meet its requirements.

Throughout Path of Exile, you'll acquire many items with colored sockets that correspond to the game's three attributes of Strength, Dexterity and Intelligence. You can socket the skill gems you find into these colored sockets to immediately gain the use of that particular skill. Each skill gem is aligned with a single attribute. For example, the Fireball skill is aligned with Intelligence, and therefore appears on a blue gem.

The color of sockets on items is strongly biased towards the attributes that the item is aligned with. For example, a Strength/Dexterity sword is very likely to contain red or green sockets and unlikely to have blue sockets. Skill gems can only be placed in a correctly colored socket.

Skill gems can be removed from sockets by right clicking on them. There is currently no penalty for doing this. We want players to experiment with the nearly infinite combinations our skill system allows.

Items can have multiple sockets, which can be different colors. Though the early items you encounter will contain one or maybe two sockets, the number will increase throughout the game. Most armor pieces can have up to four sockets. High end body armor can reach six sockets. One-hand weapons and shields cap out at three sockets, while two-hand weapons can contain six.

In addition to active skill gems, another type of gem exists – support gems. These are placed in special, linked sockets and grant additional properties to other gems they are linked with. They do not grant active skills themselves.

These linked sockets start to occur in groups on items as the player explores further into Wraeclast. Larger groups of linked sockets occur infrequently but offer greater opportunities for augmentation.

For example, linking a “Multiple Projectiles” support gem to a fireball skill gem causes the fireball skill to launch multiple fireballs when used. Linking an “Added Fire Damage” support gem to an ice nova skill gem causes the ice nova to also deal fire damage in addition to its normal cold damage.

Support gems gain experience and level up in the same way that skill gems do. Their effects increase in power as they reach higher levels. A high level “Multiple Projectiles” support gem adds more additional projectiles than a low level one.

Any skill can be augmented by any support gems where it makes sense. For example, an “Increased Area of Effect” support gem would modify:

• The size of an ice nova
• The splash radius of a fireball
• The proximity of nearby allies affected by a “Buff Other”-supported buff.

“Piercing projectiles”, “Multiple Projectiles” and various other support gems affect all skills that have a projectile component.

There are dozens of support gems and active skill gems available at launch. Many more will be added after release.

Rather than designing specific behavior for each augmentation, we have made our system extremely versatile, so that there are thousands of combinations of skills and augmentations.

The same skill can be supported by multiple support gems at once. For example, Cleave could be linked to both “Added Cold Damage” and “Increased Area of Effect” for large cleaving arcs of cold damage.

In a similar way, multiple skills can be augmented by the same support gem. If you linked a selection of different skills to “Overpower” then all of them would be dramatically more powerful and mana hungry.

The highest level skill gem or support gem in a group is picked when the skill is used. Having multiple instances of the same skill or support equipped in a group does allow you to level up multiple copies of that gem at once, but doesn’t provide extra power to the skill.

If multiple copies of the same skill gem exist in different groups, then the character receives several versions of the augmented skill, each with the appropriate supports. This means that you can have both a castable augmented version of "Fireball" as well as an augmented version of the skill that lays a Fireball Trap.

Path of Exile has a subclass of support gems that trigger their linked skills when certain conditions occur. These conditions range from scoring critical hits to becoming cursed, frozen or ignited. The triggered skills generally have modified mana costs and damage, in order to balance the automatic trigger. Complex combinations of triggered skills are possible and open up many possibilities for character builds.

We have plenty of other support gems on the drawing board that do interesting things. The “Entrap” support gem turns linked skills into traps. For example, if you link “Entrap” with Fireball and “Multiple Casts”, you get a trap that shoots a stream of several fireballs at any enemies who trigger it.