Our Kapala mala (thöpa trengwa) are made from 108 DIFFERENT skulls. I don't know of any other source that guarantees this authentic form of these tantric implements. (All others I have seen are from one or a just a few, or even from animal bone). These are mixed male and female skulls, collected for a long period of time. Because of the nature of the skull the "beads" or segments are disc-shaped, not round. They are also fairly thick—beware bone malas where each segment or bead has been split in two. The skull bone has two smooth surfaces and the inner "trabecula" or latticework of inner bone.
The kapala mala is for very wrathful practices like Vajrakilaya (Dorje Phurba), Dorje Drollo, Troma Nakmo, Hayagriva and so on. This is compassionate wrath for destroying activity for removing the obstacles to one's health, longevity, wealth, safety and especially spiritual development.
There are also the rare type that can be fabricated that has male and female kapala discs glued together—a double thick bead for special purposes.
According to tradition, the bone should be from Brahman skull or a Buddhist practitioner. These are never, ever from the very negative Chinese sources or Muslim grave-robbing sources.