A relative hardness value of 6.5 means that the mineral could scratch orthoclase (feldspar) but not quartz.

Is feldspar stronger than quartz?

Feldspars are hard minerals, all of them with a hardness of 6 on the Mohs scale. This lies between the hardness of a steel knife (5.5) and the hardness of quartz (7). In fact, feldspar is the standard for hardness 6 in the Mohs scale.

Can a knife scratch feldspar?

Feldspar Cannot be scratched by a steel knife, but it can scratch window glass.