It is quite easy to build up a large collection of Haliotidae, but very difficult to get shells in fine+ or gem condition. This is due to the many other organisms that choose Haliotis to live on. Some species, such as the Atlantic H. tuberculata is a real challenge to clean. Shells covered with barnacles will only seldom be intact. About 70 species.
For a list of genera and species:
http://www.marinespecies.org/aphia.php?p=taxdetails&id=1727