The beads used to recite mantras aiming at subduing beings through forceful means should be made from raksha seeds or human bones in a string of 60. Again, as the purpose should be absolutely altruistic, the only person capable of performing such a feat is a Bodhisattva motivated by great compassion for a being who can be tamed through no other means, for example extremely malicious spirits, or general afflictions, visualized as a dense black ball.
Bone is used in practice because the more we contemplate death the more fruitful our life becomes. We begin to live life differently and accept that death is inevitable while the path to the end of this life is about service to others, loving all, and being happy (without the pursuit of happiness) in each moment…through the supposed good and bad.
Yak is an important animal in Tibet as they are used as packing animals for the nomadic tribes and to honour their passing they are used in practice. Continuing to be of service as they pass from this consciousness to another.
Why a skull bead? Some people look at the malas with a certain curiousity when they see a swastika or a skull…I have explained the meaning of the swastika in another post.
The skull is meant to have the meditator reflect on death and impermanence and know that death is a fact and the closer that we can relate to death and embrace it the sooner we can start living with love and compassion and realizing that in this cyclic existence death will come when it is time…live life and embrace death. Acceptance is key to the inevitable passing of time and body.