Scanning a Ceramic Reference Sphere with a Ceramic Disk?

[[Ri...]]

My knowledge of ceramics is minimal at best.

Is there something odd about scanning a Sphere with a disk (as a sphere, both are ceramic material) that can cause the output in the attached image?

(See Ceramic Ø20mm Disk on Cermaic Ref Sph.JPG)

I use the same feature for a steel disk, and it doesn't have the same effect.

(Edit)

This doesn't happen when the disk stylus is at 90° to X.Y or Z. This particular setup is 45°ZX.

Edited May 5