...made me think of marble (calcite).

But that's just a first impression guess, I don't have anything to back that up. Will hunt around a bit.

You have an eye for unnoticed treasures - what is the story behind this one?

This is one of those instance when I mourn the loss of Russ Nobbs and his knowledge of the stones used in beads.

Fortunately, we still have Danny Lopacki...

At any rate, what make me think this stone is something like marble is what appears to be visible crystallization. Such stones as layered onyx agate are microcrystalline - one cannot see the mineral crystals with the naked eye.

And those spiky black inclusions in the white rim - just not typical of agate.

At first I thought it might be gabbro, of which I have a piece that resembles a panda in the demarcation of the black and white layers.
But alas, such a sharp demarcation is not typical?

Cool bead, at any rate.