Dust rune

Dust runes are runes made by combining Earth runes and Air runes. This can be achieved by taking
 * pure essence, Earth runes and an Earth Talisman to the Air altar or
 * pure essence, Air runes and an Air Talisman to the Earth altar.

These runes count as one of each type used to make it. Combinations have a 50% success rate, unless a player wears a Necklace of Binding and the talisman is consumed in the process. This is one of the combination runes released with the abyss update. The lunar magics spell Magic Imbue means that players can now make combination runes without the talisman being consumed.

Past Images
Dust Runes originally looked like, but in April, 2007 were changed to.

Reference
Knowledge Base on Crafting Runes