Where does the color of gemstones come from?

Precious stones have fascinated us for thousands of years, and a strong symbolism surrounds them. One element is particularly important: color. Thus we often speak of colored gemstones, like the stones we offer at Maison Alchimie Paris. But have you ever wondered where this color comes from? Why is a ruby red? Why is an emerald green? We suggest you dive into this ocean of colors and shades to discover the causes of the color of gems.

What is color ?

Color is the most fundamental optical property of a gemstone, and it is due to light. Light is an electromagnetic radiation characterized by a wavelength. Some wavelengths are visible to the human eye, this is called the spectrum of visible light. Natural white light is the superposition of all the wavelengths of this spectrum - all the colors of the rainbow!

Spectrum of light

When light passes through an object, especially a gemstone, some of it can be absorbed. This is called selective absorption. Thus, the color of the object that we see corresponds to the wavelengths that have not been absorbed. For example, an emerald absorbs a large part of the red light, and therefore appears green.

Chemical causes of color in gemstones

To understand where exactly the color of gemstones comes from, one must therefore ask what causes this absorption. In most cases, it is chemical elements that are involved.
Most gemstones are formed by pressure and heat: atoms are compressed deep under the earth's surface to form a stone. For example, a ruby requires aluminum and oxygen atoms. However, it can happen that other elements are trapped during this process. They are then trapped inside the stone in very small quantities. This is called "trace elements".

Many "pure" stones, without these trace elements, are actually colorless, and it is the trace elements that give them color! For example, a "pure" corundum is colorless, but a corundum with traces of iron and titanium is blue, which is the sapphire we know so well. If you replace the iron and titanium with chromium, it will be either a pink sapphire or a ruby.
It can also happen that the crystal structure of the stone - that is, how the atoms are arranged - is imperfect: one atom may be missing, another may have moved where it should not be... This is called a "color center". This is notably the cause of the color of the rare and sought-after padparadscha sapphire, a sapphire with a slightly pinkish orange color, which is said to have the color of the sunset or of the lotus flower.

Padparadscha Sapphire

Beyond chemistry

In a stone, chemistry is important, but it is not everything! This is also true for color. Colored inclusions can change the color of an otherwise colorless stone. The best known example is that of the black diamond: usually colorless, the presence of numerous graphite inclusions can give it this color so special for a diamond.

Finally, we can mention irradiation: underground, stones are sometimes exposed to natural irradiation that will alter their color. Here again, we particularly remember one diamond: the green diamond. It is also possible to artificially recreate natural irradiation in the laboratory, and thus change the color of certain stones. For example, brown zircon can become blue. To learn more, you can refer to our article on gemstone treatment methods, right here.

Dresden green, one of the most famous green diamonds.