As you may recall learning in school, there are three basic types of rocks, and they are classified as igneous, sedimentary, and metamorphic rocks. The term “rock” can include any solid aggregate of minerals or, in some cases, mineraloids.
Igneous rocks are formed from the magma that comes from under the earth’s crust – magma being hot, molten rock in a liquid form, which is itself a collection of minerals and elements from the earth. Igneous can come in two categories: intrusive and extrusive. As you can imagine, intrusive rocks form under the crust of the earth, whereas extrusive ones are formed when some part of the magma reaches an area beyond the mantle where it can cool quicker. Some examples of common igneous rocks are basalt and granite.
Sedimentary rocks are formed from the sediment, or loose deposits of minerals and organic materials (like sea-shells, skeletons, etc), that flows through water and collects in a certain spot. Sedimentary rocks cover a large amount of the Earth’s surface, but the layer is relatively thin compared to the amount of igneous and metamorphic rocks that make up the Earth’s crust. Sedimentary rocks are often laid down in layers, called strata, which makes them valuable tools in the studies of archaeology, paleontology, biology and geology. Sedimentary rocks are the only rocks that can contain fossils. Some examples of common sedimentary rocks are shale and sandstone.
Metamorphic rocks are those rocks that have undergone a change in composition or appearance due to some outside effect, usually enormous pressure or heat. Metamorphic rocks can be igneous or sedimentary rocks that have been changed by these forces, or even older, other metamorphic rocks. Contact metamorphism occurs when lava comes into direct contact with a rock, thereby changing it immediately. Regional metamorphism, on the other hand, represents a change in the rock that occurs over a large region, either through intense pressure or heat. Some examples of metamorphic rocks are marble, slate and quartzite.
How Rocks are Used
Rocks are used in many different applications, depending on the type of rock. Perhaps the best known use of rocks is in the building of structures. Humanity has been building with rocks since the aptly-described “Stone Age”. Stonehenge, for example, is made up of both igneous and metamorphic rock. The great Pyramids of Egypt were made with granite and limestone, among others. The later use of stone in castles and forts relied mostly on granite, and in some cases rudimentary bricks (which are nothing more than a solidified rock-paste).
In the modern age, bricks took over as the building material of choice, simply because bricks were easier to deal with. They could be made on-site, carted around and handled more easily. Consider that if you want to use rocks like granite, you have to physically separate them with chisels, sledges and other tools, and then haul them to your building site. This is very labor intensive! The invention of bricks allowed people to use smaller rocks and rock-dust to make sturdy building materials.
Fluorescent Rocks and Minerals
Some minerals and rocks are fluorescent, which means they give off radiation in the form of light by taking other forms of radiation and changing them. Most often, fluorescent minerals only radiate in the ultraviolet spectrum. This means that you cannot see them glowing under normal conditions, but you must instead expose them to a certain kind of energy and view them in the dark or under different lighting set-ups.
A great example of fluorescent minerals occurs in your average fluorescent light. Suspended in the light is a mercury vapor which, when hit with electricity, causes a phosphorous dust inside the lamp to glow. Fluorescent rock dust can also be added to paints, dyes and other materials so as to give other pictures and designs the ability to glow in the dark.
The Modern Use of Rocks
Many rocks nowadays are used in the building process, but mostly for their aesthetic appeal. For example, marble is used very often as a decorative choice for countertops and floors (say, in an expensive hotel) because it is smooth, glossy and very resilient. It comes in very attractive patterns and, when polished, makes a very flashy, sophisticated addition to any building set-up.
Sandstone and limestone are often used in sculpting and other artistic endeavors. Both are “softer” rocks that can be easily whittled away with tools to make intricate designs. Marble statues are still made by artists who can afford blocks of marble (marble being very expensive!) and other rocks are used in jewelry and other personal effects.
Slate and granite are used in various capacities. For example, slate is often used in high end billiards tables because of its weight and resistance to warping. Granite is often used as a cheaper alternative to marble in applications like home design, for use in counter-tops and the like, because it can polished but also has a distinctive look.
Overall there are many uses of rocks for the designing of homes, buildings and even furniture like tables. The crystalline structure of different rocks makes them suited to specific purposes. Smoother, glossier rocks tend to be valued more highly, simply because of their visual appeal. Rougher rocks have other uses, usually valued more for their utility than for their look.
Resources for Rocks and Minerals