This glass plate is about 5 on Mohs' hardness scale. You can perform a simple field test by scratching your mineral across the plate. If the mineral scratches the plate, it is harder than 5 on Mohs’ hardness scale. These plates are custom made for us by a glass maker - they swipe the edges on a grinder to eliminate the risk of accidental injury when handling. Each plate measures 2″L x 2″W x 1/4″H.