To get the most accurate results, a sharp edge should be swiped against a smooth surface, on a small an area as possible. The scratch should not be conducted on a surface that is coated, chipped, or weathered, for it will give inaccurate results. When a mineral is scratched, a permanent indentation is created. Powder of the softer mineral will come off, and it will cover the scratch area. This powder must be brushed away to see if the mineral really got scratched, or if the powder of the softer mineral that was swiped across the specimen being tested created a scratch-like marking.
All minerals are anisotropic , meaning their hardness varies in different directions.
Kyanite is famous for this habit. When scratched in one direction, it exhibits a hardness of 4 to 5. When struck from the perpendicular direction, it exhibits a hardness of 6 to 7. Besides for kyanite, anisotropism is so slight that it cannot be determined, and therefore is not considered when testing mineral hardness. Micromounts and small embedded crystals are very hard to determine in terms of hardness.