* Mohs Hardness Scale: This scale measures the relative hardness of minerals. A material can only be scratched by a material of equal or greater hardness.
* Gold's Hardness: Pure gold has a Mohs hardness of about 2.5 to 3.
* Iron Nail's Hardness: An iron nail has a Mohs hardness of around 4 to 4.5.
Since a nail is harder than gold, a nail can scratch gold.