How does a snail shell form?

A snail’s shell is a calcareous structure secreted by the mantle, a specialized fleshy tissue that lines the outer wall of the shell.12

The formation process occurs primarily along the outer lip of the shell's aperture (opening). The mantle first adds new material to the edge of the lip, extending the shell, and subsequently secretes inner layers to thicken it. The shell is composed of several layers:1
* Periostracum: The outermost layer, consisting of a protein mixture called conchin.1
* Inner layers: These are composed of calcium carbonate interlaced with a network of conchin and impregnated with mineral salts. Depending on the species, the calcium is typically deposited as calcite or aragonite crystals.1

As the snail grows, it continues this process of marginal and surface additions, typically resulting in a spiral pattern of whorls that increase in size over time.13