Rocks have been around the world for lots of years. Some rocks come from volcanoes. obsidian is an igneous rock. Igneous rocks are made of hot liquid rocks that has cooled and hardened. Liquid rock comes up onto the earth's surface through volcanoes.
Some rocks such as granite are very hard strong. Others such as chalk are very soft and easily crumble. We can write with chalk because it wears away when pressed against a hard surface.
Some rocks have tiny holes that water can flow through, other rocks such as slate do not let in any water. People make roof tiles out of slate because it keeps the house underneath dry.
Each kind of mineral forms its own type of crystal and when lots of crystals lock together they make rocks. Granite is a hard speckled looking rock, each speckle is a different crystal.
By Brody
