From the RuneScape Wiki, the wiki for all things RuneScape

A combination rune is any rune that takes two types of the elemental runes (fire, earth, air, and water) and combine them, which essentially makes two runes in one. To make such runes, players need to bring pure essence, an amount of certain elemental runes that is equal to the pure essence, and a talisman to enter the altar they choose. If the altar is not being accessed by the Abyss, then the player also needs to bring the talisman or tiara for the altar to gain access. For example, if a player wishes to make steam runes, they could bring

• Ten pure essence, ten Water runes, and a Water talisman to the Fire altar, or
• Ten pure essence, ten Fire runes, and a Fire talisman to the Water altar.

However, the chance of successfully binding the runes together is only 50%, and the talisman used to make the combination runes is consumed in the process. A worn Necklace of Binding increases this chance to 100%, though it can only be used 15 times before it is destroyed.

Alternately, players may use the Lunar Magic spell Magic Imbue to make combination runes without using a talisman. The player must use the rune instead of the talisman if they are using Magic Imbue.

The runes, in order of level needed to craft them are:

• Mist rune- Combined Air and Water Rune
• Dust rune- Combined Air and Earth Rune
• Mud rune- Combined Water and Earth Rune
• Smoke rune- Combined Air and Fire Rune
• Steam rune- Combined Water and Fire Rune
• Lava rune- Combined Earth and Fire Rune

As of now, only Lava runes, Steam runes and Mud runes have an elemental staff modeled after them: the lava battlestaff, steam battlestaff and the mud battlestaff, respectively. Being battlestaves, each of these also has a mystic version.