Minerals that have a Mohs hardness of 1 ( graphite) are the weakest ones, whereas scale 10 ( diamond) is considered to be the strongest. All grades are represented by a common mineral. Grade 2 can be scratched by a fingernail, whereas a steel knife has a hardness of 5–6. The minerals that have a Mohs hardness of 7 ( quartz) or more can scratch glass. Precious .