A mineral can also be a combination of substances that form a group of elements, such as the turquoise gemstone you see above. Here the copper has joined with oxygen and water to form turquoise. Turquoise is a mineral that contains copper.

What is a rock?

A rock is formed from a combination of minerals. An example is the blue lapis lazuli you see above. Although world famous as a gemstone, lapis lazuli is a rock. A rock that formed from more than one mineral joining together to form a group of minerals. Most minerals used for gemstones will be found in rocks. Before we begin a study of minerals, let’s learn about how rocks form. This will help us to understand better how we get gemstones, as minerals, from rocks.

Here is a look inside an igneous rock. There are two important features you should notice:

1. The rock is very porous, meaning it has a lot of holes. This was due to a lot of gas being present in the lava when it cooled.
2. The white-colored features are crystals that formed inside the lava rock as it slowly cooled. The slower the lava cooled, the larger the crystals can form due to slow cooling. This is how and where gemstones form in igneous rocks.

In the central part of Texas in the USA, there is a formation of igneous rock that is over 1 billion years old, much older than the surrounding sedimentary rocks that cover it. These igneous rocks were formed when magma tried to push up through the surface of the earth to become lava, but it became stuck and cooled just under the surface. This intrusion of magma that cooled very slowly allowed some very unusual crystals to form that are found nowhere else on Earth.

Most igneous rock formations that produce gemstones are formed by cooling magma instead of lava. The Oregon Sunstone deposits, found in the lava formed igneous rocks that we just studied, are very rare since lava cools typically too quickly for gemstone crystals to form. But in that case, the gemstones formed deep inside the earth in the magma chamber and then came to the surface with the lava.

Igneous rocks formed from magma, however, can produce large crystals of gemstones since the magma cools deep inside the earth which causes it to cool very, very slowly. This very slow cooling of molten rock allows for many gemstone crystals to form in very large sizes. It also allows for some very unusual crystal types to form since many of the ingredients or minerals found deep in the earth are rarely found on the surface. For this reason, magma formed igneous rocks will quite often have gemstones that are totally unique to any other type of formation.

Below is one very rare example.

The formation you see at left is an exposed vein of what was once molten magma deep inside the earth. When erosion exposed the formation, and the roadway was cut through it, this exposed the billion-year-old igneous formation of an igneous rock known as granite and a very rare type of crystals that grew inside this magma formation billions of years ago.

When we look closer at the rock, it has blue crystals running through it. These blue crystals are quartz crystals. Quartz is another very common mineral found in the surface of the earth, but blue quartz of this type has never been found in any place but this one outcrop that crosses a road in Central Texas. Being close to the town of Llano, it is called “llanite” and is found in only one place on earth. A rare treasure that formed deep in the earth when a chamber of magma slowly cooled.

METAMORPHIC ROCK FORMATION

We sometimes have fun with metamorphic rocks by calling them “transformer” rocks, mainly because they do transform from one kind of rock to another. Another term is to “morph”, meaning change. Yet another way to look at it is with caterpillars “morphing” into butterflies. Butterflies start out as caterpillars and, after spinning a cocoon and living in it for a while, go through metamorphosis and turn into a butterflies. They change.

The same thing can happen to rocks but in a little different method. These rocks change from one type of rock to another type due to being exposed to very high heat and pressure.

Metamorphic rocks can start out being almost any kind of rock: igneous, metamorphic, or sedimentary. Any rock that is subjected to high heat and pressure can change or “morph” into another type of rock. An example is the marble in marble floors or tables. Sedimentary sandstone that is exposed to very high heat and temperature will morph into marble, as found in the famous marble quarries throughout Italy that made that country famous for marble statues and floors. Sometimes rocks that are already metamorphic from a previous change can be changed again if exposed to heat and temperature later.

Regardless of what kind of rock was the original, metamorphic rocks can be the source of many gemstones. The reason is that a particular type of gemstone crystal may form during the morph of the rock at one stage, while another morph event in the rock can cause other gemstone crystals to form. This creates a rock that can offer many different types of gemstone crystals due to the many different times the rock has been changed from one type to another.

One of the most beautiful features of metamorphic rocks is the formations and shapes the rocks can create. While igneous rocks are formed from the cooling of molten rocks, which pretty much gives you the same appearance in most cases, metamorphic rocks offer a wide and wonderful variety of shapes and formations as the pressure and heat causes the rocks to move, curve, bend, and even twist.

The area around Mason County, Texas, offers some of the most beautiful formations of metamorphic rocks due to the intrusion of magma into the area a billion years ago. These rocks were either already here or were formed during this magmatic intrusion deep in the earth and were transformed or morphed due to the pressure and heat deep in the earth caused by the magma.

Let’s look at some of the exceptional formations that these metamorphic rocks offer.

The heat and pressure of the magma pushing against the rocks that already existed in the area caused those rocks to be bent, twisted and folded as you can see at right. These rocks became a type known as schist called “Packsaddle Schist”. The magma formed the white granite intrusion rock, while the black rock is the metamorphic Packsaddle Schist that formed due to the magma intrusion.

Above is another look at a section of this formation in Mason County, Texas. You can see the white granite intrusions. This contained hot water that was full of minerals that cooled to form natural blue topaz crystals found in this metamorphic rock. We will cover that in a later chapter. But the critical part of this image is that the intrusion of the hot, molten magma into the previously existing rock caused the rock to “morph” into the black schist you see here.

Here is another excellent view of the white granite intrusion and the black metamorphic rock that it caused to form. This all occurred deep underground over a billion years ago. The area was exposed over time due to erosion, and eventually, the Texas Department of Transportation cut through the formation to build a road. Road cuts are very important places to find unusual rocks.

Close to this intrusion is another interesting formation where the pre-existing rock was twisted and bent. Notice the lines in the rock at right. These were originally straight formation lines in this host rock, but the intrusion of magma a billion years ago heated the rock to almost molten and created so much pressure on the rock that it bent and twisted.

Above is an excellent view of the bending of the original rock formation. It is difficult to imagine the amount of heat and pressure that had to be put on this rock to make it soft enough to bend and twist in the manner shown by these images.

Here is another dramatic view above of this formation and the bending of the host rock that occurred during this magmatic intrusion. The pressure and heat required to turn these rocks into what would appear at the time to be like plastic is enormous.

The image above is one of the most famous formations in this location. Here you can see that the Packsaddle Schist has been subjected to such heat and pressure from the intrusion of the magma that it folded the rock backwards. This rock layer was once flat. But the magma pushed so hard into the space this rock was already taking that the magma caused the rock to bend back as you see here.

Many gemstones can form in one of these metamorphic formations since the heat and pressure will melt certain minerals and allow them to cool very slowly. This slow cooling of the minerals can allow for large crystals to grow in the metamorphic rock. Above, you see quartz veins that run through this Packsaddle Schist metamorphic rock. This was due to hot water being in the magma when it pushed into this rock. The hot water created a healthy environment for gemstone crystals to grow.

Above, you see large quartz crystals that grew inside this igneous rock when it was molten magma. The area where you see these crystals is the area where other crystals can grow and are often found. These include rubies, sapphires and many others. The pink rock around the edges of the quartz is feldspar, another type of gemstone when found in specific types.

Finally, one more area of large quartz crystals in this metamorphic rock formation above. The magma had hot water that was full of all types of minerals in the mix. As the hot water cooled the minerals formed crystals based on what types of minerals were present. You can clearly see how these quartz crystals grew inside the metamorphic rock.

Let’s take a look at the Packsaddle Schist metamorphic rock.