Igneous Rocks: These rocks form from the cooling and solidification of molten rock (magma or lava). Crystals are the building blocks of most igneous rocks. Here are a few examples:

* Granite: A common igneous rock with large, visible crystals of quartz, feldspar, and mica.

* Basalt: A dark-colored volcanic rock with smaller, often microscopic crystals.

* Obsidian: A volcanic glass that forms when lava cools so rapidly that crystals don't have time to grow. It's technically not crystalline but is still an igneous rock.

Metamorphic Rocks: These rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are changed by heat, pressure, or chemical reactions. Many metamorphic rocks are composed of crystals:

* Marble: Formed when limestone is metamorphosed. It has interlocking crystals of calcite.

* Slate: Formed from shale, it has very fine-grained crystals.

* Gneiss: A banded metamorphic rock with distinct layers of different minerals, often including quartz, feldspar, and mica.

Sedimentary Rocks: These rocks are formed from the accumulation and cementation of sediments like sand, clay, or shells. While some sedimentary rocks may contain crystals, they are not generally made up of them.

* Sandstone: Formed from cemented sand grains. While individual sand grains can be crystals (like quartz), sandstone isn't a crystalline rock.

* Limestone: Often formed from the accumulation of shells (made of calcium carbonate), but it's not a crystalline rock.

Key Point: The size of the crystals in rocks varies greatly. Some rocks have large, visible crystals (like granite), while others have microscopic crystals that are only visible under a microscope.

Let me know if you'd like to explore any of these rock types in more detail!