From Bulbapedia, the community-driven Pokémon encyclopedia.
Volcanic ash (Japanese: かざんばい Volcanic ash) is a type of currency in Pokémon Ruby, Sapphire, and Emerald. It can be collected on Route 113 and Jagged Pass, where it settles from Mt. Chimney, covering the tall grass. The player must first obtain the Soot Sack, which is used to collect the ash from the ground by walking through the grass. The grass will have a gray appearance if there is still ash on it. After the player walks through it, it will turn green. Collected ash can be taken to the Glass Workshop on Route 113, where it can be used to make either glass flutes or furniture. The furniture will be sent directly to the PC and can be used to decorate a Secret Base.