The stupa represents the mind of the buddha, and enshrines his physical relics, and is a hand held attribute of several deities. The hand held stupa is white or golden in color, and is actuele specified for these skulls, they are usually represented in a rotary of twelve, sixteen, or twenty one skulls.
Robert Beer, The Handbook of Tibetan Buddhist Symbols, Serindia

> Ceremonial Wandering
> I am the soul in limbo
> Nameless Swirls, an Unfolding in Presence