A... ghrm. We don't really know how that system will work, but logic dictates that every creature will start out with default materials for flesh, bones, and all applicable tissues. If your question is of the "what if I call a material belonging to the creature from another material belonging to the same creature" variety, then my answer would be that you will get a default material if the material in question has not been initialized yet, or the actual material used for the creature if that material was already defined by that point. It would be an obvious loophole to plug, and Toady would probably think of that.
I presume the actual question you were trying to ask is "what would happen if I wanted to make a borgle, with bones made of borgle's borgle". The answer is: depends on whether the borgle's borgle is initialized before or after the borgle's bones; in one case you get a default material with average properties and a default name, in the other you get the borgle's borgle.
Initialize: dwarf lungs, material default_dwarf_flesh. Result: Success.
Initialize: dwarf pancreas, material dwarf bone. Result: Warning, no such material. Defaulting to default_dwarf_flesh for pancreas. Success.
Initialize: dwarf skeletal structure, material default_dwarf_bone. Result: Success.
Initialize: dwarf left eyeball, material dwarf bone. Result: Success, dwarf bone defined by dwarf skeletal structure.