The fundamental problem is that rune essence was one of the few areas of the game which could be used to generate a reasonable amount of RuneScape money on a 'throwaway' low level free account. Normally to earn large amounts of money quickly you need to level up an account first, and then of course you care if it gets banned so our normal deterrent is effective.

We have therefore split rune essence to have 2 grades. Normal essence can only be used to make the six basic runes, whereas pure essence can be used to make any rune. You can mine both types of essence at the same place as before. The kind of essence you receive is automatically decided when you mine the rock. You will always get pure essence only if your mining is over 30 and you are on a members' world, otherwise you will get normal essence.
More...