Ceramic Calibration Sphere?
Check this site out (looks like a load of balls to me...)
All Probe Characterization Spheres
The choice of material for the master calibration sphere can be of great importance, depending on the contact force of the measuring probe and its spherical diameter. The important factor in the choice of the material for the master calibration sphere is its stiffness. When the probe contacts the master calibration sphere, there is considerable Hertzian elastic deformation of both the master calibration sphere and the spherical contacts of the probe. The amount of these deformations depends mainly on the probe sphere diameter and the contact force of the measuring probe. The smaller the probe ball, and the higher the measuring force, the more elastic deformation will occur. If we use a very stiff material like ceramic or tungsten carbide for the master calibration sphere, and then measure ordinary materials such as aluminum or steel; we will loose appreciable accuracy. This built in error will be greater with the high measuring force typical of many of the modern scanning probes and will be higher for small diameter probe spheres than for the larger ones. The nearer the stiffness of the master calibration sphere matches the stiffness of the part being measured, the less error will result. For this reason, steel calibration spheres have been the order of the day until recently.
If the master calibration spheres are replaced in kind, all standard diameters of ceramic and tungsten carbide master calibration spheres are available.
Now then! Can anyone expand on this caption for the pic:
Two Calibration Spheres walk into a bar....the barman says...