From Ohio History Central
The color of the powder of a mineral. If the mineral’s hardness is less than 7, the powder can be obtained by rubbing a piece of the mineral across an unglazed, unpolished porcelain surface. (Plumbers often can supply unglazed porcelain tiles.) If the mineral has a hardness of 7 or greater, then it probably will be necessary to scratch a small part of the specimen’s surface with a sharp instrument that has an equal or greater hardness.