Runite ore

Runite ore can be obtained through the Mining skill in various places throughout RuneScape by mining a Runite rock. It requires level 85 Mining and grants 125 Mining experience. The ore can be smelted with 8 coal through the Smithing skill to form a Rune bar, which can then be smithed into various rune weapons and armour.

Runite ore is the most difficult ore to obtain as it is usually found in the Wilderness or other high-risk locations, has very few available rocks to mine from, and it has an exceptionally long respawn time of 12 minutes, prompting most players to use the "World hop tactic" while mining.

Dropping monsters

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Respawn rate
As of 17 July 2014 and according to the Boss Pets and Spawn Rates update:

That means rune ore spawns at a standardised spawn rate, which is 12 minutes after it was last mined, no matter how many players are on a world. Whether it's zero or 2000 players in a single server, all ore across the game will respawn at the same rate.

Other way of obtaining
Another way of obtaining Runite Ore is through the Motherlode Mine. One with a mining level of 85 or higher can wash pay-dirt and have a small chance of receiving runite ore along with 75 experience.