Use the simple field test below to identify the relative hardness of a stone. Stones in the range of 6.5 to 7.5 will usually polish well.
- Fingernails scratch 2 to 2.5
- Pennies scratch 3.0
- Knife blades scratch 5.5
- Window glass scratches 5.5
- Steel files scratch 6.5
- Garnet scratches 7 to 7.5
- Carborundum scratches 9.5