Limestone is a rock made mostly of the mineral calcite. The grains of calcite are normally very small and can't be seen with the naked eye. If the limestone is heated or put under pressure, the calcite grains can melt together and form larger calcite crystals. Marble is a metamorphic rock. Most rocks are made up of crystals.
Granite is a good example.
It forms when molten rock, many kilometres below the surface of the Earth, cools slowly . As it cools, crystals of quartz, feldspar and mica grow. You can see these crystals very clearly with the naked eye although they may not display the regular geometry we associate with crystals. In this case, geologists tend to call them grains. Granite is called an igneous rock because it forms from molten rock.