What are those bracelets called? (Read Description)?

Last year, me and my friend went to some carnival.. I bought this cool bracelet that was like an ankle bracelet and made of thin rope.. It had a little seashell in the middle. The guy that sold them to us said that "When you put them on, tie them in a knot and make a wish and when the knot comes undone, your wish comes true." Does anyone know what those bracelets are called? I lost mine in my room and I think I threw it out. :/