Well you use runes to unlock spells.
For a player at or above level 91 Magic, unlocking all the Lunar spells requires a total of approximately 2785 rounds or 557 cycles. This means the following runes are required,[1] which are worth approximately 13,813,000 gp with mud staff, 13,891,000 gp with lava staff or earth staff or 16,573,000 gp with no staff. An earth staff in combination with a tome of frost can also be used. Players below level 91 Magic require more time and runes.
54,029 Astral runes
23,951 Nature runes
3,342 Law runes
11,140 Water runes
297,995 Earth runes
-from rswiki
I unlocked all the spells and i love the exp so much i kept going.