Essence of Ice

is the fourth of the 5 available essences in the game. It is used for focuscrafting and alchemy. Harnessing essences of ice can only level the player's Energy Harnessing level up to a maximum of 201. The crystals have a chance of dropping Essential Mana instead of the essence.

Source
Dropped by monsters and crystals from the following provinces:
 * Ostgarath
 * Serpentis Isle
 * Fyrmount ( Blazing Vale area )
 * Westonia

Best place to farm is probably Frigid Vigil in Mount Tyrannas.