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.
There are other igneous rocks which do not have such large crystals. Basalt is an igneous rock which forms from lava which cools on the surface of the Earth. The crystals in basalt are difficult to see without a microscope, because the rock cooled quickly. The crystals did not have time to grow large. Another igneous rock, obsidian, cools so quickly that no crystals form at all.