You can tell how hard a material is by a simple test.

For this test you need a nail, a copper coin and your fingernails.

If a material can be marked by your fingernail it is soft.

If a material cannot be marked by your fingernail, but can be marked by the edge of a coin, it is medium hard.

If a material cannot be marked by the edge of a copper coin or by scratching at it with a nail then it is very hard.