The term for a mineral's resistance to being scratched is its hardness. The hardness of an unknown specimen can be measured using the Mohs hardness scale, which compares the mineral being tested against 10 different reference minerals.
Determining a mineral's hardness is important because this is one of the key tests needed to identify an unknown mineral. This is because Mohs hardness scale is usually very reliable, as most specimens of a specific mineral have quite a consistent level of hardness. This makes it fairly easy to accurately determine the origin of a specimen by simply trying to scratch its surface with another mineral.
The Mohs hardness scale was developed by German mineralogist Friedrich Mohs in 1812. Mohs selected 10 different minerals, ranging from extremely soft talc to an extremely hard diamond to develop a system by which other minerals could be compared to the 10 reference points to see where the specimen falls on the hardness scale.
Basically, if the specimen can scratch the reference mineral, it is harder than that mineral. However, if the reference mineral scratches the test specimen, it means the specimen falls below it on the hardness scale. If neither mineral scratches the other, they are the same hardness.