There are 3 main rock types.

Sedimentary Rocks
Sedimentary rocks are formed on or near the Earth’s surface from pieces of other existing rock or organic material (plants / bones). E.g. sandstone, form from clasts, or pieces of other rock; coal, formed from hard, biological materials like plants, shells, and bones that are compressed into rock: limestone is created through chemical precipitation (rain or water droplets dripping) calcium or salt in the precipitation is deposited once the water evaporates.

Metamorphic Rocks
Metamorphic rocks are rocks that have been changed from their original form by immense heat and pressure. Often you can see multiple colours or wavy mixed layers within the rocks as they have been forced together under extreme pressure. Examples include: slate, gneiss (pronounced “niece”) and marble.

Igneous Rocks
Igneous rocks are formed when molten hot material cools and solidifies. Granite is an example of igneous rock. It has a coarse texture with large mineral grains, indicating that it spent thousands or millions of years cooling down inside the earth.

Obsidian cools into volcanic glass so quickly when ejected that the grains are impossible to see with the naked eye.

Pumice is created when Volcanic magma erupts and cools on the surface leaving lots of vesicular or holey textures. When the magma cools gas pockets get trapped inside as the magma hardens (solidifies) leaving a bubbly texture like an Aero bar.