Conjure Mana Gem is a core mage ability learned at level 47. It conjures a mana stone that stores mana for later use. Mana stones are soulbound, and cannot be traded to other players. They are unique, meaning that a mage can only carry a single stone at a time. When used, the stone instantly restores mana, similar to a manapotion with its own 2 minute cooldown.
Don't wait until you're out of mana to use a stone. In a fight that will be more than two minutes long, use your first charge early in the fight, after you have used enough mana to use the stone and not over-restore mana. A second charge will be available before the end of the fight.
Since all potions share a cooldown timer, you can use this instead of a mana potion that would put the health (and mana) potion on the cooldown timer. If you always uses your mana stone before your mana potion, you have quicker access to your health potion.
/Patch 3.0.2 (14-Oct-2008): Conjured mana gems no longer disappear from your backpack after being logged out for more than 15 minutes. Higher ranks of Conjure Mana Gem will recharge an existing mana gem to maximum charges.
Patch 2.3.2 (2008-01-08): Conjure Mana Gem mana restore variance substantially reduced, (Rank 5 – Emerald) now restores 2340 to 2460 mana and has three charges.