The 411 on..ROCKS!
The three MAIN categories of rocks are igneous, sedimentary, and Metamorphic. Two examples of an Igneous rock are Obsidian and Tuff. A way you can remember the word Igneous is that it sounds like ignite, which is to "set on fire", which is kinda how this rock (Igneous) is made. Igneous rocks are formed when magma within earth cools and gets hard. Now for some examples of a Sedimentary rock. Flint, Coal, and Limestone. Sedimentary rocks are formed when bits of things ( such as shells and sand ) get packed together. This process can take millions of years. A lot of rocks on the ground are Sedimentary. Last but not least, we have Metamorphic rocks. Some examples of Metamorphic rocks are Slate and Marble. THESE rocks are made by Igneous AND Sedimentary rocks that are heated and pressured together deep under the Earth's crust.
Rock [ rok ] noun: A mineral matter of variable composition, assembled in masses or considerable quantities in nature, as by the action of heat or water.
AND THERE YOU HAVE IT, THE 411 ON ROCKS!!!!! : )