There are so many different crystals and gemstones that exist in nature, and collectively they have an incredibly vast range of colors all over the spectrum. Some types of gemstones can have several different colors, like Topaz, Quartz or Tourmaline. So how do they get their colors?

In their chemical structure, gemstones have trace amounts of different transition metals either as part of their chemical compound or as impurities. Transition metals are the metals that make up the large, middle part of the Periodic Table, and these metals are able to absorb colored light. Different wavelengths of visible light can be absorbed by different transition metals, leading to the different colors that we see. 

Purple Amethyst - colored by the irradiation of Iron ions in place of Silicon

For example, the purple coloring of Quartz that we call Amethyst is caused by the irradiation of Iron ions in place of Silicon in some locations of the structure.

Garnet’s red coloring is caused by Iron ions replacing Magnesium ions, and the lovely blue variety of Beryl that we call Aquamarine is from the presence of Iron ions replacing Aluminum ones.

Citrine

Emerald

Opal

Quartz w/Hematite

The yellow color of Citrine - another form of Quartz- is caused by Aluminum or Iron impurities.

In Emerald, Chromium ions replacing Aluminum creates green. In Ruby it creates red!

Topaz in it’s pure form is colorless, while atomic level imperfections cause the blue, yellow and brown varieties. 

Topaz in it’s pure form is colorless, while atomic level imperfections cause the blue, yellow and brown varieties.

Transition metals are not the only way that gemstones get their colors, however. In some stones, such as Sapphires, it is caused by the transfer of electrons between ions.

 Color can also be caused when an ion in a specific place is missing within the structure, or diffraction of light through the structure, like in an Opal.  

Simple inclusions of tiny minerals inside a larger solid crystal can change its color. For example if a clear crystal like Quartz or Apophyllite or Calcite has red Hematite in it, the color of the crystal itself can appear to be red.

These are the most common reasons for the colors of crystals, or at least their colors as they appear to the human eye. There are more reasons, for more complicated crystals such as Alexandrite, Labradorite and some Fluorites. Crystals are complicated and the more you learn, the more complex they become!

Natural crystals are formed in many ways, though the overall process is generally the same- a very hot substance slowly cools, creating a solid. Many crystals are formed when molten rock from a volcano cools down very slowly, allowing the atoms to arrange themselves into highly organized, repetitive geometric patterns enclosed with flat outward faces called facets.

When there are ideal conditions and enough space to grow, the crystals will have smooth faces and showcase their perfect shape. In contrast, when many crystals form together, they become a large mass with no faces or crystal shapes. This is how most rocks form, and the term for these shapeless rocks is ‘massive.’ For example, Quartz- one of the most common minerals on the surface of the Earth- is formed by crystallization of molten magma. Quartz occurs in both; as beautiful crystals with smooth geometric faces and in massive form.

Quartz is formed by crystallization of molten magma

Crystals start out very small and ‘grow’ as atoms are added. They can form from water that has a lot of dissolved minerals in it, they can form from melted rock as described earlier, and they can even form from vapor. As a crystal grows in size, the atoms attach more easily to the rough parts than the smooth ones, creating larger smooth planes until the entire crystal is encased in these smooth, flat faces.

The characteristic external shape of a crystal is called it’s habit. A crystal’s habit is determined by the crystal structure as well as the conditions of the environment when the crystal formed. Crystal habits are used to describe the shape that a crystal has formed in, and one mineral can occur in several habits.

For example, Fluorite can be Octahedral, having eight-sided octahedrons, shown on the left above. It can be Cubic, or cube-shaped, shown in the center photo. Fluorite can also be Botryoidal- exhibiting grape-like hemisphereical masses, shown on the right. These are three of Fluorite’s habits. You can easily look at them and determine the habit of these Flourites without any special equipment or tests, which makes it a popular way for rockhounds to describe and classify minerals. 

We know that molten rock, or magma, must cool very slowly if it is going to form crystals- so what happens if it cools very quickly- for instance if it reaches the air on the surface? This is when magma becomes lava- and lava cools quickly when it reaches the surface, into a natural glass-like rock called Obsidian. Obsidian can be black, Mahogany, Rainbow, or even have shimmery gold and silver sheen in it, caused by various inclusions in their composition. 

Magma becomes lava- and lava cools quickly when it reaches the surface, into Obsidian

These are just a simple few examples of the ways that crystals form, there are many variations and combinations of crystals that form in an endless number of specific ways but I hope by scratching the surface you can see just how amazing and fascinating the Earth's crystal treasures are!

An Epimorph occurs when one mineral forms on another one, and then the first mineral is dissolved by acidic liquid or gas, leaving a cast of the original mineral in the latter one. 

Epimorph from Turkey of Quartz

Here is an epimorph from Turkey of Quartz (var. Smoky and Amethyst) that formed on another mineral - probably Calcite. Calcite is softer than Quartz, or lower on the Moh’s scale of hardness, therefore it is dissolved more easily. Because of this, the Quartz was able to remain intact while the Calcite was dissolved away, leaving this beautiful cast on the bottom of this piece. 

In this epimorph from Colorado, the white Quartz has formed on scalenohedral (aka “dogtooth”) Calcite, and the Quartz was left intact as a perfect cast of the spiky, cone-like Calcite crystals.

A pseudomorph is when a mineral replaces something else, keeping the shape of the initial substance rather than its own. Petrified wood is a great example of a pseudomorph, in which the organic material of the wood has been gradually replaced by silica until the entire branch or log is stone, in the exact shape of the wood. It can even retain the grain of the wood and the bark that was on the tree.

Petrified Wood (Pseudomorph)

Many fossils are pseudomorphs - Ammonites are another example, as the organic material of these creatures has been replaced over time by Silica, Calcite, Pyrite et cetera.

Many fossils are pseudomorphs - like this Ammonite

Another way that pseudomorphs occur is when one mineral replaces another mineral. Here is a pseudomorph from Pennsylvania of Goethite after Pyrite, or Goethite that has completely replaced Pyrite and kept the shape of the original mineral now that it is gone. The epimorphs that we first looked at are also considered pseudomorphs, they are called ‘incrustation pseudomorphs’ because they assumed their ship by encrusting the initial mineral rather than replacing it. 

Pseudomorph from Pennsylvania of Goethite

As you can see, minerals can be sneaky, shape-shifting bastards. But don’t worry, it’s just one more reason to love them. They are complicated, interesting individuals and the more I learn the more I realize there is to learn about them! I love to share what I learn and I hope you enjoy it too.

Quartz broken from it’s place of formation

Sometimes Quartz that has broken from its place of formation, through tectonic movement or extreme temperature changes, can ‘heal’ itself in the right conditions. Self healing or Re-healing is when the Quartz continues forming over the fractured section of the crystal to form new faces and terminations. Here is a Quartz crystal that has begun to heal, you can see the beginnings of the geometric crystal faces taking shape.

Quartz crystal that has begun to heal itself

Sometimes a piece that has separated heals so well that you can’t tell where it separated at all, this is referred to as a “floater,” and many are highly prized collector’s pieces like this Smoky Quartz, which has formed many stepped, triangular terminations over its fracture.

Smoky Quartz “Floater” that has healed itself