You might find this article on “orient” interesting (I think it's been cited elsewhere on the forum):
http://www.opticsexpress.org/DirectP...TOKEN=95789648
But I think you might be referring to “overtone.” I believe Strack says that (natural, not chemical) overtone depends on nacre thickness and the size and arrangement of the aragonite platelets.
There are some discussions of bleaching on the forum, but I think in this case what you are seeing probably is more dependent on the pearl's structure, shape and bodycolor. The reason I say this is because certain types of pearls generally have more pronounced and varied plays of color than others (even if unbleached). But perhaps the experts can chime in on this!
Perle