I'm not real sure what you mean by the molt thing either...
Like Kat12 said, as far as I know, pretty much all molluscs (snails, clams, oysters, nautiloids, abalone, etc.) grow their shell from the inside - the mantle secretes the nacre(I think that's the word ?), and builds up the shell from the inside and forward edge - why they have the spiral shape.
Also why you see the points eroded/worn down on some MTS kept in soft/acidic water - the snail's mantle isn't in contact with that part of the shell, so it can't repair it, and the water keeps dissolving it.
As to the outer layer with the colors and such, I think that's organic, but I don't really know anything about how it's produced or what not.
Anyways, you're snails do look great - I love how the stripes match up across the spiral of the shell. neat little critters, and cool that you can find them locally.