Mud rune

Mud runes are combination runes. They count as two separate runes: one Water rune and one Earth rune. Thus, any spell requiring one Water rune, one Earth rune or both will spend only one Mud Rune.

Runecrafting Mud Runes
At Runecrafting level 13 or higher, they can be crafted in either of two ways:
 * Pure essence, Water runes and a Water Talisman to the Earth altar or
 * Pure essence, Earth runes and an Earth Talisman to the Water altar.

Note: For the combining to work you must click one of the already made runes onto the altar (e.g. click a water rune on the earth altar).

Players receive 9.3 experience for combining Earth runes at the Water altar and 9.5 for combining water runes at the Earth altar. Combinations have a 50% success rate (or 100%, if the player is wearing a binding necklace), and the talisman is consumed in the process (unless the player uses the Lunar spell Magic imbue).

Miscellaneous
Mud runes are used in the quest The Eyes of Glouphrie.

As Monster Drops
Living rock protectors and Living rock strikers in the Living Rock Caverns drop Mud runes. As do also Molanisks very rarely.

Trivia

 * As with all combination runes, the most amount needed for a spell is 30, for the Charge Orb spell series.