Which glue should I use to bond sea shells to steel and/or stone? What kind of glue should I use to glue sea shells to steel and sea shells to stone?  These will  be displayed outside.  I have tried a hot glue gun but it doesn't do well outside.
Depending on how ‘shock-proof’ you need it to be, you have a couple options: silicon or epoxy.

Silicon is a bit more flexible, so if people or pets are going to be touching it, then this is what I would recommend. It is waterproof and really sticky until it dries. My advice is to get the transparent type used in bathrooms.

Two-part epoxy is theoretically stronger, but it might actually be too strong and might not bond well-enough with the steel.