Harmonic dust is used to make crystal tools and equipment by Crystal Singing. To make the items you need either a crystal weapon seed, crystal armour seed, or crystal tool seed along with a certain amount of harmonic dust. Use the singing bowl in the Ithell Clan district and sing the item that you want, which gives Smithing experience equal to the amount of dust used. You can also pay Eluned or Ilfeen 1,000,000 coins to make any crystal item for you (except the pickaxe and hatchet).