Answer is: a. harder.
Friedrich Mohs devised a scale that tested mineral hardness, which means the resistance of a mineral to being scratched. This scale (from 1 to 10) is known as the Mohs Hardness Test.
For examples:
fingernail (hardness = 2.5).
copper penny (hardness = 3)
glass plate or steel knife (hardness = 5.5)
steel file (hardness = 6.5).
According to this scale hardness of the mineral is between 5.5 and 6.5.
