The African elephant's misfortune has been its teeth, in particular the well developed pair of upper incisors known as tusks for which it is being killed in large numbers. The visible ivory part of the tusks is made up of dentine with an outer layer of enamel, and when viewed in cross section it reveals criss-cross lines that form a series of diamond shapes. This is what gives the elephant ivory its distinctive lustre, and makes it so valuable economically.