Here's why:
* Crystals form over time: Crystals grow slowly as minerals precipitate out of a solution or melt. The longer they have to grow, the larger they can become.
* Conditions matter: The specific conditions during crystal formation affect their size. Factors like temperature, pressure, the availability of space, and the chemical environment all play a role.
* Types of rocks: Large crystals are more commonly found in igneous rocks (formed from cooling magma or lava) and metamorphic rocks (formed when existing rocks are transformed by heat and pressure).
* Examples:
* Quartz crystals can grow to be very large, like the famous "Crystal Cave" in Arkansas.
* Geodes are cavities within rocks lined with crystals, often containing spectacular large specimens.
* Pegmatites are igneous rocks that can contain extremely large crystals, like the giant feldspar crystals found in Madagascar.
So, while not every rock contains large crystals, it's definitely possible to find them! They can be a beautiful and fascinating part of geology.