3mm stone-grade tungsten carbide rounded chisel stylus for the ZPT-MA The Maia air scribe. The diameter is engraved on the stylus. 

For use with the 3mm head. 

The styli we use in our Maia are fitted with a locking mechanism to prevent rotation. This means you can sharpen the point, and if you do sharpen it off centre it won't matter. 

Stone-grade tungsten carbide is by far the best material for fossil preparation – it withstands the abrasive nature of rock and stays sharp far longer than other metals like steel which will quickly blunt and wear down to nothing. However, tungsten carbide can be brittle. A stylus can last many years without requiring replacement when looked after correctly.