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. Collected ash can be taken to the Glass Workshop on Route 113, where it can be used to make either glass flutes or furniture.
While volcanic ash only exists in Ruby, Sapphire, and Emerald games, some flutes can be obtained from non-player characters in Pokémon Platinum and Pokémon HeartGold and SoulSilver, as well as the yellow flute in FireRed and LeafGreen from the Game Corner.