Explanation:

• Petrology is science of rocks. A petrologist studies rocks in all their aspects viz., mineral composition, texture, structure, origin, occurrence, alteration and relationship with other rocks.
• The earth’s crust is composed of rocks. A rock is an aggregate of one or more minerals.
• Rock may be hard or soft and in varied colours.
• For example, granite is hard, soapstone is soft. Gabbro is black and quartzite can be milky white.
• Rocks do not have definite composition of mineral constituents.
• Feldspar and quartz are the most common minerals found in rocks.

Source: NCERT Class 11