To understand the formation of colored gemstones you should first know about the places in which colored gemstone are formed. That is, the types of formations that present the right combination of temperature, pressure, and elements to allow colored gemstone crystals to form. We can identify these regions by the rocks that are present in the area. Here are the basic:

Igneous Rocks

These are formed by volcanic intrusion into an area. And may contain a number of different gem crystals. This Texas Pink Granite is used for many purposes, but is well known for the large feldspar crystals that grow as part of its basic structure, which gives it the characteristic pink color. Gemstones that will be found in basic igneous rocks include peridot, diamond, garnet, ruby, and sapphire to name a few.

Sedimentary Rock

Once igneous rocks are worn down by erosion they form sediments on the ocean floor, which are then covered by more and more sediments, until extreme pressure and heat is built up. This causes the sediments to form what is known as limestone rock as shown in the photo at left. These layers of rock represent literally millions of years of sedimentation that was pressed into the various rock layers you see at left.

Within the layers of sediment you can usually find a variety of fossils of both plant and animal life. You will also find a special rock, used to make arrowheads by the Native American Indian Tribes, which is the result of ancient sea sponges being transformed into fossilized rock known as flint.

 

Metamorphic Rocks

During the course of time the sedimentary rocks will become compressed to the point that they will change into other type of rock. As shone by the black schist rock shown at left. This is a result of extreme pressure and temperature literally baking the rock to change its chemical structure. When this occurs many gemstones are formed during the change of structure. Some of these include ruby, sapphire, chrysoberyl, emerald, and others.

If you will note in the photo at left there are two vertical pink sections running through the black schist. These are known as pegmatite dikes and are some of the most important gem producing formations in the world.

 

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Pegmatite Dikes

These are igneous rock intrusions into either sedimentary, metamorphic, or igneous rocks, that melt the rock in the contact zone where the intrusion touches the host rock. As you will notice from the three photos above, the first shows two small white lines running through the rock next to YourGemologist's Jeep Wrangler. The second photo shows a closer look at one of the two rather thin pegmatite intrusions, while the third shows a close up. You will note that where the two colors meet is the general growing area for many of the colored gemstones that you know. In this particular case, the intrusion is too thin to form gem crystals of any size since is cooled so quickly. However, I have found tiny topaz crystals in this pegmatite dike located in Mason County, Texas. Pegmatite dikes will produce gemstone crystals of topaz, tourmaline, chrysoberyl, amethyst, citrine, and many, many others.

How Are Colored Gemstones Found

Once the crystals have grown (please see page on Crystal Systems) they are either mined by digging them out of the rock, as was done with the tourmaline crystal in the photo at left. It was extracted from a pegmatite dike as shown above. Or, they erode and are washed into river or stream beds to be found by washing the river gravel. These deposits are known as alluvial deposits and produce water worn pebbles. Many sapphires, rubies, diamonds, and spinels are found in this manner. The other type of water erosion deposits are those which were once in a water environment but are now found on dry land since the river is no longer active. These deposits are known as elluvial deposits, and will still produce water worn pebbles even though they are found on dry land.