Greater Magic Essence

Greater Magic Essence is a stored form of concentrated magical energy used in Enchanting. A Greater Magic Essences may be disassembled into three Lesser Magic Essences.

Source
One way to obtain Greater Magic Essence is by disenchanting items in the 16 to 20 level range, more often from weapons than from armor. Above level 15, it can only be obtained from weapons.

Another way to obtain Greater Magic Essence is by right clicking the Lesser Magic Essences, that will turn them into the Greater ones (conversely clicking on one Greater Magic Essence will break it into 3 Lesser Magic Essences). It takes 3 Lesser Magic Essences to make 1 Greater Magic Essence.

Enchanting

 * Enchant 2H Weapon - Lesser Intellect
 * Enchant Bracer - Minor Agility
 * Enchant Chest - Lesser Mana
 * Enchant Cloak - Minor Protection
 * Enchant Weapon - Minor Beastslayer
 * Enchant Weapon - Minor Striking
 * Greater Magic Wand
 * Runed Silver Rod