Re: CMM calibration recommendations
CMM probe tips are usually made from artificial ruby or Silicon Nitride (a type of carbide that is mostly used when scanning Aluminum castings). I have never seen plastic used.
It's not necessarily damage that you're looking for, the sphere could be thermally expanding/contracting and not be correct without any visible damage.
Now I'll be somewhat honest, unless you are concerned about micron level tolerances you could ignore it to save some money, but it's $150 for a calibration and I have mine on a 2 year cycle.
Also, (on Calypso/Zeiss, possibly others) if you use disc shaped probes you will want a certified ring gage for setting them.