Rage crystal is a meter, like mana. Some skills require certain amount of Rage Crystals to cast. It is displayed at the top right corner of the screen for Warriors and Rangers. Magicians do not have a Rage Crystal meter.
How to Obtain Rage Crystals
Rage crystals are gained every time you make an successful attack (not blocked or missed). For each normal damage, you gain 0.5 Crystal (2 hits for 1 Crystal). For each critical or deadly damage, you gain 1 Crystal.
Warriors also gain Rage Crystals from taking damage. For each damage instance you receive you gain 0.2 Crystal (5 hits for 1 Crystal).
Damage from skills such as Wild Swing counts toward your Crystals. However the amount of Crystals gained is irrelevant to the amount of damage dealt. Double Shot, Multi Shot and Sweeping Strikes count as multiple damage instances.
None of magicians' skills require Rage Crystals to cast.
It is possible to extend the maximum amount of Crystals you can have. You start the game with a maximum of 2 Crystals, and this limit increases by 1 for every 10th level (10, 20, 30, 40, 50). At level 50 you will have 7. With Instinct Stimulus Vol.1, you will have 8. As for now, only Instinct I exists.
The skill Instinct Stimulus increases maximum amount of Crystals.