It is a passive skill when gear with gems is equipped.
Each hero owns 3 gem skills, and one of them,which is unlocked, can be chosen to be used.
To activate gem skills, you should unlock all slots for the skill with Secret of Hero items, and have the required gems on your equipped gear.
Gem skill level is decided by the least number of gems present on your gear that is required for the skill. (Max Lv. 3)
When you unequip or change gear, the skill would be deactivated if there is not appropriate amount of gems on equipped gear.
※ Gem Converting Stone, which can be obtained from Honor Shop, events and missions, can convert the gem on your gear.