Actually, it's neither.
The handles on these knives are what Case calls 'Staminawood', which is their trade name for jigged laminated wood. By 'laminated', that means it's in very thin layers sandwiched together like plywood and impregnated with a resin. Makes them extremely durable and essentially waterproof. They are frequently mistaken for jigged bone, and often advertised as such on the 'auction site' and other places. The resin in the laminate tends to fill up the grain that would otherwise make it more recognizable as wood, and it takes on a shinier finish, almost (but not quite) as smooth & shiny as polished bone.
BTW, Case DID handle some older Folding Hunters in bone, especially back in the 1940-'64 era and earlier. Pretty sure none of these Mariner Set knives were done with it, as they've all featured the 'standard' knife with the Staminawood handles.
David
Wow, thanks, David! I had not known about that! It certainly makes sense, and would certainly make the knife more practical in environments that may be damp or wet, and as you say, it's very durable.
Jim