The bone that grows out of the back of Delenn's head is exactly that, not
a decoration, but an actual part of the physiology. It will differ with
various Minbari in size, configuration and texture (another Minbari seen in
the show has a darker tint to the bone structure, it's cracked and so on).
The thought is that it grows from the back of the skull structure, and comes
forward with age. You can see the "root" of thed _ the _ structure whenever
Delenn turns around.

jms