Preserving a Shell

Hello, my beautiful giant african, passed away the other day. They're body is buried in a pot (a giant teacup) and the shell has been gently brushed clean in water.

I'd like to put in in a safe little display case next to the urn of another pet, how can I ensure the shell wont go yucky? And preserve their stripes and colour?

I've heard of people oiling the shell, what kind of oil would one use?

Their name is Ghost, because when they were a lil baby climbing the glass they looked like a lil spirit flying away. Later I thought of Ghost in a Shell and felt quite clever. I miss my lil guy so much.

Any help would be greatly appreciated, thank you.