Feldspar hardness varies depending on the specific type of feldspar. The most common feldspars, orthoclase, plagioclase, and microcline, all have a Mohs hardness of 6. This means that they can be scratched by quartz (Mohs hardness of 7) but not by a steel knife (Mohs hardness of 5.5). Some less common feldspars, such as amazonite and labradorite, can have a Mohs hardness of up to 7.