Waco Gem and Mineral Club, Waco, Tx

10.1 What is a Rock


10.1: What is a rock?

Minerals are inorganic substances with unique chemical compositions created in nature. Inorganic means they‘re not alive. Minerals often produce crystals, and a particular type of mineral always has the same chemical make-up that gives it a distinctive crystal form and color/s. Minerals are the individual units or building blocks that, brought together, make up a rock. Rocks are inorganic solids from the earth’s crust that are made up of one or more minerals. To provide a comparison for kids, you might say that everyone in your club represents an individual mineral. You have boy minerals, girl minerals, mother and father minerals, etc. Scattered around town, each is an individual, but when you bring them together in the same room, the individual boys and girls and parents become something new: a rock club. Just so, when individual minerals come together in a group, they create a rock.

Granite is a good example for showing how rocks are made of collections of minerals because crystals of the individual minerals making granite are especially large and visible as compared to some other types of rocks. Although different types of granite will have different combinations of minerals, most granite is made of the minerals feldspar, quartz, mica, and hornblende. The quartz will tend to be clear or milky and shiny like glass. The feldspar might be white, gray, or pink and somewhat dull. The mica will appear as silver or black glittery flakes. And the hornblende will appear as black specks. Have your kids examine a specimen of granite under a magnifying glass to see the different types of minerals in order to gain an appreciation of how a rock is made up of different minerals that have grown together.

Rocks are divided into three groups:

Igneous rocks cooled and crystallized from hot, molten magma, either on the surface of the earth or deep below ground. Igneous is derived from the Latin word igneus, meaning fire. Examples of igneous rocks your kids might collect include granite, basalt, rhyolite, obsidian, gabbro, tuff, andesite, pegmatite, or pumice.

Sedimentary rocks formed by gravel, sand, or mud that got buried and hardened due to pressure from overlying rocks. Sedimentary rocks start by processes of erosion that create gravel, sand, or mud that settles to the bottom of a basin (ocean, lake, or river valley) in layers. These layers eventually harden to become conglomerate, sandstone, or shale. Sedimentary is derived from the Latin word sedimentum, which means to settle or sink down. Sedimentary rocks also include those that precipitate out of water, either through chemical action or evaporation, such as limestone, gypsum, or halite (salt). Examples of sedimentary rocks your kids might collect are shale, sandstone, breccia, conglomerate, limestone, coquina, diatomite, dolomite, travertine, or gypsum.

Metamorphic rocks are pre-existing rocks that have been altered by extreme heat and/or pressure to create a rock with a new form and mineral structure. Metamorphic is derived from the Greek word metamorphōsis, which means to change or to transform. Examples of metamorphic rocks are marble, gneiss, slate, schist, quartzite, soapstone, greenstone, and serpentine.

Note: Kids can use this activity to satisfy requirements toward earning their Rocks & Minerals badge (Activity 1.4) and Collecting badge (Activity 5.1) simultaneously.