The property you are testing by rubbing one mineral against another is hardness.

Here's why:

* Mohs Hardness Scale: This scale ranks minerals from 1 (softest) to 10 (hardest) based on their resistance to scratching.

* Relative Hardness: When you rub two minerals together, the harder mineral will scratch the softer one.

* Testing: You can use a known mineral of a specific hardness on the Mohs scale to determine the relative hardness of an unknown mineral. For example, if a mineral can be scratched by a fingernail (hardness ~2.5), but not by a copper penny (hardness ~3.5), you know its hardness falls somewhere between 2.5 and 3.5.

Let me know if you'd like to know more about the Mohs Hardness Scale!