Hardness. The ability to resist being scratched or hardness is one of the most useful properties for identifying minerals. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties.

It's not reliable as the other physical properties since some objects may have the same color as it has, or it may look similar to other stuffs. That's why the hardness is the easiest physical property of mineral.

Answer:

The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral's hardness, are more useful for mineral identification. Color is readily observable and certainly obvious, but it is usually less reliable than other physical properties.