Active Skill Gems
Active skill gems, or known as skill gems are an item class which grants active skills to the characters by placing them into an item socket. Similar to Support Gemss active skill gems can gain experience and level up increasing both their attribute and level requirement as well as the effect of the stats granted by the skill.
Active skill gems can be further augmented by placing appropriate Support Gemss into a linked socket.
Active skill gems have gem tags which are used to categorize them, but note that gem tags are not intended to indicate whether an active skill gem can be supported by other skill gems or whether specific stats (or modifiers) apply.
In term of item mod and passive skill, such as the mod +2 to Level of all Minion Skill Gems from Unnatural Strength, the "skill gem" refers to Active skill gem only. Support skill gems are refers as "support gems" in passive skill and item mods, such as Catarina, Master of Undeath's +# to Level of Socketed Support Gems.