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