This is a multi-tip star stylus that can be used where contact with different surfaces is needed, without changing the stylus.
The ruby tip is very hard wearing and is ideal for most standard applications.
A silicon nitride tip is recommended if you are scanning aluminium as particles will not be deposited on the surface, unlike with ruby.
Zirconia tips are more suitable for abrasive surafces, such as cast iron parts.

Renishaw A-5000-3609 M2 ø2 mm ruby ball star styli centre, stainless steel stem, L 16.3 mm