Explanation: 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.
• 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.

Source: NCERT – Fundamental of Physical Geography