Final answer:
Approximately 5,575 different minerals are known according to the International Mineralogical Association. Most rocks are composed of just a few common minerals.
Step-by-step explanation:
Approximately 5,575 different minerals are known according to the International Mineralogical Association. However, most rocks are composed of just a few common minerals. In fact, only seven minerals make up 84% of Earth's crust. So, when you find an unknown rock, there is a good chance that at least one of those seven minerals will be present.