Runite ore

Runite ore is a type of ore that can be mined through the Mining skill in various places throughout RuneScape. It is the rarest and most valuable ore in RuneScape. You need Level 85 to mine the ore (3,258,594xp), and it sells for around 11,000 - 12,000 coins, making it one of the fastest and best ways to make money.
 * In free worlds, there are only 2 runite rocks located above the lava maze in the level 46 wilderness.
 * Member's worlds boasts 5 other rocks;
 * 2 located in the Heroes' Guild basement,
 * a third one in the black dragon lair being guarded by level 64 poison spiders and 2 black dragons
 * a fourth near the Dark Beasts under West Ardougne on the way to the Light Temple as featured in the Mourning's Ends Part II quest.
 * One in the center of the Fremennik Isles (reward for The Fremennik Isles quest).

It can be smelted with 8 pieces of coal through the Smithing skill to form a rune bar, which can then be smithed into various types of weapons and armour. Runite ore takes around 12-24 minutes for it to respawn depending on the population of the world, which is why many people who mine runite like to world switch.