Lava runes are combination runes. They count as two separate runes: one Fire runeand one Earth rune. Thus, any spell requiring one Fire rune, one Earth rune or both will spend only one Lava Rune. They can be purchased from the Mage Training Arena, North of the Duel Arena.
Level required to craft lava runes is 23. It can be done in two ways:
bring pure essence, fire runes and a fire talisman to the earth altar
bring pure essence, earth runes and an earth talisman to the fire altar
"Use" the talisman on the altar. Tiaras can't be used in place of talismans while crafting combination runes. The talisman is consumed during crafting, unless the player uses the Lunar spell Magic imbue.