Deepstone is a rare mineral found only at the bottom of the ocean. Its unique properties make it the only solid substance that can affect a ghost. Similar in some chemical respects to water, the molecules of Deepstone are always moving. Some theorize that while ghosts can move through solid objects, they cannot pass through something that is always in motion. The ninja wear armor made from Deepstone as protection against possession.

Ninja Dojo
Elemental Blade
4.5 Bricks
Inventing Gadgets