Sulphur smell inside shell

[Fighting fish]

I had a shell which absolutely stunk of a sulphur like smell, really strong, any ideas what this could have been? Many thanks

 

[Wyomingite]

It probly had a bit of the dead snail left in it and didn't have any oxygen circulating within, allowing sulfur reducing bacteria to proliferate due to anaerobic conditions within the shell. These bacteria would have created hydrogen sulfide gas, the source of the sulphur, rotten egg smell. The same thing happens in the substrate of aquariums when there are dead zones in the tank.

WYite