Application / Research
It is important to know the hardness of specific minerals, especially were jewelry is involved. If the mineral is too soft, it will chip or if it comes in contact with something that is abrasive, it will scratch. If you were having a ring made, it would be a good idea to use diamonds, rubys, emeralds, sapphires, amethysts and opals.
If you are using the minerals in metal structure, it is important to use the strength you need. Soft metals will bend, hard metals will be expensive and take more time to construct.