How to Identify Different Rocks

By eHow Hobbies, Games & Toys Editor

When dealing with rock collecting, there's often more to it than meets the eye. Different types of rocks reveal different stories in history. For example, rich fossils often lie in the layers of sedimentary rocks commonly found near bodies of water, while lava can be an excellent source for metamorphic rock. Knowing how to differentiate the three rock types is a great place to start when you want to know how to identify different rocks.

Know the Basics

Know the three types of rock--sedimentary, igneous and metamorphic.

Study the origin of the rock. Igneous rock forms beneath or at the Earth's surface due to the cooling and solidifying of molten materials. Granite, solid volcanic lava and basalt are examples of igneous rock.

Learn how to identify metamorphic rock, which was once one form of rock but has changed to another. It forms due to heat, pressure or some other influence all the while remaining in a solid form. Examples are marble, which comes from limestone, and slate, which results from shale.

Know what lies in your local riverbed. Sedimentary rock forms through the deposition and solidification of sediment such as clay. Usually it is transported by water (rivers, lakes and oceans), ice (glaciers) and wind. These rocks may appear in layers and are known to be rich in fossils. Limestone and shale are examples of sedimentary rocks.

Identify Different Rocks

Examine the specimen closely. Begin by looking for differences in texture like solid, porous, crystallized or grainy elements.

Pour vinegar over the porous or smooth rock. If it sizzles, you have a common sedimentary rock known as limestone or possibly a metamorphic rock known as marble. If not, you will need to further examine the rock for answers.

Note the arrangement of crystals, the color of its holes or the size of any particles. Small particles are usually sedimentary rock like sandstone, while large particles are another sedimentary rock called conglomerate.

Pay special attention to rocks with holes, as they can fall into either of two categories. Light-colored holes represent a sedimentary rock called tufa, while dark holes are surprisingly found to be igneous rock such as pumice.

Check the surface of the rock. Is it glassy, shiny or dull?

Observe the layers within a layered rock. Although sedimentary rock is known for its layered characteristics, several types of metamorphic rock also reveal layers such as slate and schist.