Mud runes are combination runes. They count as two separate runes: one Water runeand one Earth rune. Thus, any spell requiring one Water rune, one Earth rune or both will spend only one Mud Rune.
Mud runes are also ground to make Ground mud runes, a secondary ingredient for Extreme magic potions, which accounts for their relatively high value. Grinding the rune awards the player with 1 Herblore experience.
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).
Tiaras can't be used in place of talismans while crafting combination runes. 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. One talisman will convert all pure essence in the inventory.
A talisman isn't required if the Lunar spell Magic imbue has been cast recently.