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