Wasn't there the example of a world made out of pieces of cosmic eggshell affecting map mechanics?
fakedit, found a reference:
Yes, but this only means the map logic has to be capable of handling cosmic egg shells and the like, i.e. provide a mechanism for those to be placed on the map (as well as to grow the map around them, when appropriate), and it's quite possible Toady will define them in his usual way, i.e. such that they can be generated procedurally using a set of rules for the outcome. However, this just means the mechanism to place such things has to be present, not the capability of actually generate any of them (and, in fact, the mechanism to place them doesn't have to be present either, only the hooks required to do it).
I'd expect such a mechanism to be capable of using multiple generated inorganic materials (and possibly organic ones too, and probably ordinary ones as well, such as e.g. gold metal [as well as gold bearing ore]), together with rules for how to place it (a big shell, a cube, shattered pieces of different sizes, etc.). While the resulting map engine may be capable of placing cosmic egg shells on maps, the "current" world wouldn't generate them.