Types of cameo shells

This is the most commonly used shell in cameo carving. It is a Red Helmet shell or
Bull mouth shell, sometimes called a carnelian shell 
The correct name is Cypraecassis rufa

This is also a very commonly used shell for cameo carving. It is a Emperor Helmet shell or Queen Helmet shell, sometimes called a Sardonyx Shell
The correct name is Cassis madagascariensis

Most white shell cameos you will see are carved from this type of shell, these carvings are commonly misnamed as "angelskin coral" which is far more rare.
This is a Queen Conch shell or Pink Conch shell
The correct name is Lobatus gigas