Rocks vary in hardness and can be ordered from soft to hard, as is shown in the following table.

Collect some rocks of as many different kinds as you can find. Number the rocks, and make a list of the numbers. Now, to determine which rocks are hardest and which are
softest, see if one rock will scratch another. A rock that scratches another rock is harder than the other rock. Using your list of numbered rocks, list them again in alphabetical order, to set up your own hardness scales for rocks.

If you can learn the names of the rock-types you have collected and tested for hardness, then you can list granite (for example) as being harder than sandstone.