Why do scarlet hermit crab shells look like rock?

Why do the shells look like rock or like they are completely encrusted with something?

Most hermits such as blue legged hermits, have shells that look the same as when the snail had it. Every image I find on scarlet hermits show them in what looks like an old fossilized shell.

No other hermit that I have found shows any images of a similar shell.

Here is a good image of mine hanging out of his thick rock-like shell.

http://img375.imageshack.us/img375/9807/scarletcra...