This is because they cooled and solidified very quickly on or near the Earth's surface, giving the minerals little time to form large crystals.
Here are a few examples:
* Basalt: A dark, fine-grained volcanic rock common in oceanic crust.
* Rhyolite: A light-colored, fine-grained volcanic rock rich in silica.
* Obsidian: A glassy, volcanic rock formed when lava cools so quickly that crystals don't have time to form.
It's important to note that even though these rocks have no *visible* crystals, they are still composed of minerals. However, the minerals are so small that they are only identifiable using a microscope.
