An unknown mineral scratches glass but will not scratch quartz. Which measure on the Mohs scale shows its hardness?

The best measure of the hardness of the mineral is orthoclase = 6.

For glass, Mohs hardness = 5.5.

For quartz, Mohs hardness = 7.

The hardness of the mineral is somewhere in between.

The best we can do is say that the hardness of orthoclase = 6.

glass is on the Mohs scale 5.5 and quartz is a 7 so the number is 6