Rocks are mixtures, or aggregates, of different minerals. They are divided into three categories based on how different rocks are formed: igneous, sedimentary, and metamorphic.
Igneous rocks are formed when hot magma from volcanoes is rapidly cooled, either by hitting underground air pockets or by flowing from the mouth of a volcano as lava. Granite, obsidian, and pumice are all common examples of igneous rocks.
Sedimentary rocks are formed by layers of earth being mixed and compressed together for extended periods of time. Common examples of these rocks are limestone, sandstone, and shale.
Metamorphic rocks are a combination of rock types, usually compressed together by high pressure, which tends to give them a more hard, grainy texture than the other two types. Schist, slate, and gneiss (pronounced like ‘nice’) are metamorphic rocks.
If you or your children are unfamiliar with what these rocks look like, you may want to examine at least one specimen from each category. Our basic set of 35 mounted rocks and minerals is ideal if you cannot find many specimens on your own. Also, if possible, compare two or more rocks per category, so that you can observe differences and similarities. For example, pumice has a very rough, porous surface and obsidian, another igneous rock, is very smooth and shiny.