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.
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!