An Introduction to Identifying Rocks and Minerals

Naturally occurring inorganic materials like rocks and minerals have long fascinated human beings.

Naturally occurring inorganic materials like rocks and minerals have long fascinated human beings. Whether you are studying the history of a geological region, identifying necessary supplements to maintain a healthy human body, collecting colorful stones, or determining which precious gemstones to pass down to your descendants, rocks and minerals have been an important, longstanding, and closely intertwined part of human history. However, the important differences between rocks and minerals can be vague or confusing to those unfamiliar with the subject. This article will break down the differences between rocks, minerals, and tumbled stones to better clarify the unique properties and qualities of each.

Classification of Rocks

Simply put, a rock is a solid accumulation of one or more minerals. Every rock is composed of different combinations and amounts of various minerals that continually change over time through temperature and pressure. As rocks are not uniform in composition, they therefore cannot be classified by their makeup. Instead, rocks are classified by the ways in which they were formed through natural processes. Geologists and other earth scientists have categorized rocks into three different classifications, which are outlined below.

The first category of rocks are called igneous rocks. With its root “ignis” coming from the Latin word for fire, Igneous rocks are generally hard rocks formed within volcanoes or deep within the Earth’s mantle and crust where the temperature is extraordinarily hot. Igneous rocks begin as magma, which is molten rock. Magma is less dense than solid rock, causing it to rise toward the Earth’s surface. Igneous rocks can be further segmented into extrusive or intrusive igneous rocks. Extrusive igneous rock is the result of lava reaching Earth’s surface and experiencing the cooling and hardening process. Common examples of extrusive igneous rock are lava rocks, obsidian, and volcanic ash. Intrusive igneous rocks are typically formed far underground when magma is introduced to preexisting layers of rock and is then, over time, thrust toward the surface. Examples of intrusive igneous rock are granite and basalt.

The second category of rock is called a sedimentary rock. The most common type of rock, sedimentary rock is the aggregate of compacted sediment within water that has hardened over time. Sedimentary rock is generally formed when small bits of rock and minerals are broken down through a process known as weathering. Thus, sedimentary rock typically has a layered appearance. These pieces of rock settle at the bottom of a body of water and slowly form into one solid piece of new rock through cementation, compaction, and recrystallization. Some examples of sedimentary rock are sandstone, limestone, and shale.

Finally, the third category of rock is called metamorphic rock. The word “metamorphic” has its origins in the Greek language and means “to change form”. Metamorphic rocks begin as igneous or sedimentary rocks and undergo a physical and or chemical transformation through immense heat and pressure that results in a new rock. Requiring factors like heat and pressure to alter its composition, metamorphic rock is typically found well within the Earth’s crust and is often harder than both igneous rock and sedimentary rock. Mountain chains are often made up of metamorphic rocks, and the presence of metamorphic rocks in hilly or flat plains suggest the area was once witness to mountains that have since eroded away.

Metamorphic rock can be formed in one of three ways: contact, regional, or dynamic metamorphism. Contact metamorphism occurs when magma comes in contact with existing rock, raising its temperature and producing a non-foliated rock, or a rock without cleavage or a banded appearance. Examples of non-foliated rocks are marble, hornfels, and quartzite. Metamorphic rock produced through regional metamorphism are subjected to tremendous amounts of pressure that result in bent and broken foliated rocks like gneiss and schist. Dynamic metamorphism includes large amounts of both heat and pressure that cause the involved rocks to bend, fold, flatten shear.

Classification of Minerals

Minerals are inorganic, naturally occurring solid substances with a fixed chemical structure. Minerals can be made from a single element, like gold, or from a combination of elements, like salt (composed from the chemical compound sodium chloride). The difference between rocks and minerals is found in the composition of minerals and the way that minerals are scientifically categorized. Minerals have specific chemical structures that allow them to be classified by their chemical makeup, while the composition of a rock throughout its structure is almost never consistent. Earth is home to over 2,000 minerals, but only 30 of those minerals make up the majority of rocks on this planet. Similarly, while there are 92 naturally occurring elements on Earth, only eight elements are found in the vast majority of minerals. Those eight elements are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

Minerals are often divided into two subgroups: silicates and non-silicates. Silicates, or minerals that contain silicon and oxygen, make up over 90% of the Earth’s crust. Minerals that do not contain these two elements are all simply classified as non-silicates. Examples of non-silicates are calcite, table salt, sulfur, and copper. In addition, minerals can be categorized based upon their physical structure. Commonly found minerals often bear a crystal structure, which is formed by a repeating, evenly spaced three-dimensional pattern of atoms, ions, or molecules. However, minerals that are not found with an orderly, crystalline structure are known as amorphous solids.

Minerals are graded within a specific set of criteria through the properties they show. Properties like luster, hardness, streak, cleavage, specific gravity (density) and color all help mineralogists and geologists decipher the mineral they are looking at. While hardness, density and color are self-explanatory terms, streak, cleavage, and luster may be unfamiliar to some. Luster describes the ease in which a mineral reflects light. Streak describes the color of a mineral in powdered form, which can be determined by creating friction between the mineral and a hard surface. Cleavage defines the way in which a mineral breaks into smaller pieces, which are commonly either small cubes or thin sheets. Minerals with low or no cleavage tend to easily fracture, or break at random places instead of consistent planes.

Minerals are necessary for healthy organic life, but have also facilitated remarkable human invention. Humans have leveraged the properties of minerals to support every day activity of peoples across the world. For example copper, a non-silicate mineral, is the most vital mineral used in modern life. Powering items from toasters to cars is made simple through wiring containing copper, a natural conductor of electricity. Sulfur, a single element mineral, is integral for making fireworks and matches. Gypsum serves as the basis for drywall and acts as an initial line of defense against building fires due to its chemical makeup which includes water.

What are tumbled stones?

Tumbled stones are rocks and minerals that have been polished until their surfaces become smooth. This transformation turns them into bright, colorful pieces of natural beauty that come in a variety of shapes and colors.

Stones are tumbled in a device called a “rock tumbler.” This container takes the rocks and turns them, a process that takes up to several weeks, against an abrasive surface and water as it smooths the surface of the stone. Eventually, the stone is then turned with a rock polish, such as aluminum oxide, to add a gleaming finish. After this process, the stones can then be treated further with dyes, oils, wax or heat. These additional treatments enhance existing colors or patterns and can improve the appearance of the stones by hiding imperfections.

When choosing a rock to use in a tumbler, it’s important to choose a stone between a 5 and 8 on the hardness scale. A stone too hard may not polish as well, and one too soft may not be durable enough to endure the process. Common materials that people often use include agate, granite, minerals like garnets or moonstones and varieties of crystal quartz. Some people have even used small pieces of petrified wood.

Tumbled stones have a wide variety of personal and commercial uses. For someone just getting started with their own rock collection, tumbled stones are an easy and cost-effective way to begin acquiring unique pieces. Tumbled stones have historically been used as tools for meditation and spiritual wellness. Commonly found in spas, these stones are often referred to as healing crystals, and in some alternative medicinal practices, are used to treat the body’s spirit or “chakras.” In a more secular sense, tumbled stones can also be used in home decor, jewelry making and other crafts.

Conclusion

Though they are prevalent virtually everywhere and naturally occur on Earth, the wide expanse of rocks and minerals continue to captivate humankind. Understanding how to identify and categorize rocks helps us to decipher Earth’s history long before human beings were the most populous species, and knowledge of minerals helps human beings to support healthy organic body functioning and propel the ecosystems in which naturally occurring minerals are found.