Rune Essence have no mining requirements, however may only be gathered after completing the quest Rune Mysteries. After level 30 mining, all essence mined at the Rune Essence mined is called Pure Essence.
Both Rune Essence and Pure Essence grant 5exp when mined.
When Pure Essence is examined, it says: An uncharged Rune Stone of extra capability.
Rune essence are used to create runes via Altars. This is used through the Runecrafting skill.