Different mollusks all have different types of shell. No two shells are the same. 

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A mollusk’s mantle (skin) releases liquid shell materials, which harden on contact with water or air. Gastropod and nautilus shells grow from their outermost edge. As the mollusk grows, its shell develops more whorls (single turns in a spiral shell) or chambers. In bivalves, new shell material is deposited on the edge that is farthest from the hinge.
Mollusk shells are made of a chalky material called calcium carbonate. The shell has three layers for extra strength: a tough outer layer, a chalky middle layer, and a shiny inner layer, next to the animal’s skin. The shiny layer in some bivalve mollusks is known as mother-of-pearl.
Although shells are common for mollusks, not all have shells.