11. Will it be easy to scratch off the earphone shell?

The hardness of metallic gold (Au) is 2.5 (Mohs scale of mineral hardness), which is similar to that of human nails. The higher the purity, the lower the hardness.
Gold purity refers to the content of gold. Generally, K is used to indicate the purity of gold. 24K gold has the highest purity, and 18K the second, etc. The FDX is plated with 24K gold, so the coating is very soft and easy to scratch off.

If a metal needle (such as a pin) is used to scratch real gold, it will leave obvious nicks. So we should take good care of it.