This is the M4 Ruby stylus #A-5003-4792.

Straight styli are designed to inspect simple features where direct, unobstructed contact with a measured surface is possible. A tungsten carbide stem provides exceptional rigidity, particularly for styli with small ball and stem diameters. Ruby is regarded as the industry standard for stylus tips. It is one of the hardest materials available and suitable for most applications. Due to adhesive pickup, ruby tips are not recommended for scanning aluminium parts.