Here's why:

* Hardness is a material's resistance to scratching or indentation.

* When a harder material rubs against a softer one, the harder material will leave a scratch on the softer one.

* Since the diamond scratched the silver, it means the diamond has a higher resistance to scratching than silver.

This principle is used in the Mohs Hardness Scale, which ranks minerals from 1 (softest) to 10 (hardest) based on their ability to scratch other minerals. Diamond sits at the top of the scale, making it the hardest naturally occurring material known.