This picture shows hollow chissels that was made for around 6000 years ago during the stoneage. The hollow form are grinded to its form. We so not know exactly how - but we think that wooden rods was made, smoothen, and that they use fat and Quartz sand as Abressive - and LOTs of time to do the job.. They use this rod as we use a big file - we think - but perhaps they was able to rotate the rod and hold the stone against the rod when it rotate. That would give more meters grinding surface per minute - and also a fixed angle.
The stone is a veey hard type of stone, not soft stone, becouse the chissel must hold for its work to hollow sifferent things in wood.
Thomas