Does anyone know the title or author of this fantasy book?

OK. The main charecter's name is Ariel and she has red hair. (Not the Little Mermaid.) She sails across a sea of dust and finds a lump of ambergris on the shore. I think the dust was blue. While at the sea she finds this shrimp thing that later turns into a glowing erfly while she's in a dark tunnel. She falls down a hole in into a dwarf(?) kingdom. On the cover of the paperback I had there's a girl with red hair holding a lute. That's about all I remember. I read this books years ago and I would really like to find it again.