Customizing the Materials of a Geometry Collection piece (bone)

Hi,

I have a Geometry Collection, but I would like to customize the materials of a “piece” (bone) of the geometry collection.

At first I thought, I will add a second material to the Mesh and then fracture it.
So I did that. As can be seen the different materials are nicely assigned. But the inside of the bone is still the “default material” (rocky material).

I was wondering if it is possible to assign a custom material to individual bones. I noticed the “AutoUV screen” and played with it. But I haven’t found a way yet.

Any help would be greatly appreciated.