Ivory isn't a gem/stone, but it's as close as Death gets to having one (and it's fairly suitable, too). It's considered an organic gemstone.
Anyway, this is a clever observation. I'd always sort of had this vague notion that each element had a specific gemstone associated with it, but I never really consciously thought about it.
Maybe...
Gravity - Sapphire
Light - Gold
Looks like Time, Aether, and Air need gems, too.
