Well, i will say that personal, from my own preferences as a Western collector, i am less interested in exactly what kind of ivory a hilt might be made of than i am the quality of the carving and the condition and patina of the piece. Elephant or hippo, i could care less really. It does seem to me that marine ivory (a term i do accept and usually assume is whale tooth since most dungong doesn't seem to have the mass to allow for the carving of many hilt forms) tends to age to a nice warm orange color that i do really like, but beyond that i really have no ivory preferences.
