Have you ever wondered what the rocks in your backyard are made of? How about the lead in your pencil? There is a scientific field devoted to answering these types of questions called mineralogy. Mineralogy is the study of minerals, how they are formed, and what they are made of.
There are more than 4,000 different types of minerals, according to the International Mineralogical Association. That's a lot of minerals! For a substance to be called a mineral, it must have five basic characteristics. The substance must be a solid, it must have a definite internal atomic structure, it must be non-living, it must have a chemical composition that changes within specified limits, and it must be a natural phenomenon (occur naturally). Minerals are divided into different categories based on their chemical makeup. The most common form of minerals is silicates. Silicate minerals are mostly made up of oxygen and silica in the form of silicon dioxide (SiO2). More than 90 percent of the naturally occurring rock formations on our planet are made up of silicate minerals.
Not all rocks are common. There are some, known as gems, that are precious because they are pretty as well as rare. Almost all gemstones are minerals that have been cut and polished. Most gems are crystals like diamonds or rubies, but some aren't. Substances like jet and amber come from organic, or once living, sources. Jet comes from decayed wood that has been under extreme pressure, and amber is fossilized resin from trees. There are many kinds of gemstones, and classifying them can be difficult. For example, a red type of corundum is called a ruby, but every other color is considered a sapphire. There are no established scientific ways to divide gemstones, so they are generally classified by species, group, and variety.
Gems are precious because they are rare, and for centuries, people have believed there was a connection between the month you are born in and specific gems. There are people who believe your date of birth determines the kind of gem that is best for you. Did you know that the belief in birthstones goes back to the first century? The first known connection between the months of the year and specific minerals comes from a historian named Josephus. He believed there was a connection between the 12 stones of Aaron's breastplate, the 12 signs of the zodiac, and the 12 months of the year. So not only do you get a sign all your own on your birthday, but you get a mineral, too!
There have been many gems associated with the different months throughout history. In 1912, the National Association of Jewelers created a standardized list so everyone can know their particular birthstone, and that is the list followed by most jewelers today, with a few additions and subtractions.
Types of Rocks
The type of mineral you've probably had the most contact with are the rocks outside. A rock is a solid substance that has a specific chemical makeup. There are many kinds of rocks and many different ways to make them. Some are made by pressure and others by heat or layering. There is even a rock cycle, which illustrates how rocks are formed and can change from one type of rock to another. The three types of rocks are igneous, sedimentary, and metamorphic rocks.
Igneous rocks are rocks made from lava and are further divided into two types, plutonic and volcanic, based on how the lava cools. Volcanic rock is made when lava cools quickly, which prevents it from forming a definite crystalline structure. One of the most common types of volcanic rock is pumice. Plutonic rock is made when lava cools slowly, which gives it time to form a crystalline structure. One of the most common types of plutonic rock is granite.
Sedimentary rocks are formed by the layering of sediments like sand or gravel. Each new layer of sediment puts weight on the layer under it and presses the layers together. As time passes and the pressure on lower layers builds, they undergo cementation. Sandstone is a common form of sedimentary rock made of grains of sand fused together through cementation.
Metamorphic rock is rock that is made from other rocks. When igneous or sedimentary rocks are exposed to high pressures or temperatures, the minerals that make them can undergo chemical changes. These changes affect their crystalline structure and can happen when the original rock is exposed to a naturally occurring heat source like lava or is buried. Metamorphic rock can even be made of other metamorphic rocks!