Answer: He found a rock, and the crystals inside are minerals.
Options:
He found a rock, and the crystals inside are minerals.
He found a mineral, and the crystals inside are rocks.
He found a rock, and the crystals inside are rocks, too.
He found a mineral, and the crystals inside are made of dirt.
Explanation:
A rock differs from a mineral. While they are both solid, a mineral has a crystalline structure while a rock does not have a specific structure. Rocks are made up of different mineral structures and that is what Joseph discovered on dropping the rock, one of the component minerals in the rock. For example, a type of rock, slate may consist of feldspar, quartz among many other minerals.
