Lava rune

Lava runes are runes made by combining Earth and Fire runes. This can be achieved by taking
 * rune essence, Earth runes and an Earth Talisman to the Fire altar or
 * rune essence, Fire runes and an Fire Talisman to the Earth altar.

These runes count as one of each type used to make it. Combinations have a 50% success rate and the talisman is consumed in the process.

Reference
Knowledge Base on Crafting Runes