Glass plates are great for testing the hardness of minerals. Glass plates are a key item on the Mohs Hardness Scale and will help you determine the identity of your mineral
samples. To perform the hardness test, place the glass plate on a solid surface
then rub the mineral on the plate. If the mineral scratches the plate, the
mineral’s hardness is greater than 5.5 on the Mohs scale. If the mineral does
not scratch the plate, the mineral’s hardness is equal to or less than 5.5. Each
glass plate is 2” x 2”.