One day at the beach me and my husband were lucky enough to find a bunch of (empty!) conchs at the beach. (It's illegal to collect live ones).
My husband then boiled them in water for a while and afterwards he let them soak overnight in a water & bleach solution. After that he spent some time (read: hours) carefully picking of the barnacles and other stuff that were stuck to the outside of the shells. After that he polished them with mineral oil, and: VOILÁ!