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Types of Stones in a Rock Collection

There are literally thousands of types of rocks on Earth that have been in existence and changing form for billions of years in some cases. Although the number of elements that make up most rocks are few, the combinations and physical processes over time have created a wide range of different and easily identifiable rocks. Many students, children and geology enthusiasts collect rocks for fun or for educational purposes. A good rock collection may have any number of stones in it, but almost everything found in a rock collection will be in one of a few groups.

Igneous
- Many rocks in a rock collection are in a group known as igneous rocks. The various cooling rates and depths of molten rock, or magma, beneath Earth's surface, form igneous rocks. The rocks that form when they cool deep inside Earth at a slow rate often grow large crystals that cause a coarse-grained rock, while fast-cooling magma at Earth's surface, such as that ejected from a volcano, form tiny crystals and are smooth. The stones that result from this cooling magma are igneous rocks. Obsidian is a fine-grained, very slick and black stone that is one example of an igneous rock. Granite is also an igneous rock that forms deeper in the surface with large crystals and coarse grain.
Sedimentary
- Sedimentary rocks are formed at the surface of Earth or on the floor of bodies of water. Sedimentary rocks are the result of different sediments, such as dust, sand, animal remains or other terrestrial components bonding together in layers to form a rock over time. The layers of sedimentary rocks are generally parallel to Earth's surface where they were formed. If they are distorted, then the earth has moved at some point during the formation of the rock. Layers of sand can form sandstone. Layers of compacted mud can become shale.
Metamorphic
- Heat and pressure can transform rocks substantially, even to the point of making them into a completely different type of rock. These new rocks are metamorphic rocks. They were previously igneous or sedimentary rocks, but circumstances and time have caused them to become something new. According to the U.S. Geological Survey website, the limestone in the Guadalupe Mountains in West Texas were a coral reef in a tropical sea 250 million years ago. The limestone is therefore a metamorphic rock.
Minerals
- Many rock collections contain pieces of minerals that may have been part of another rock. These minerals are often unusual looking and are popular for rock collections because of their distinct appearance or other characteristics. Among the most common examples of minerals found in rock collections include quartz, potassium feldspar, olivine and calcite.

Rock Collecting