Building A Palette Using Candy-Colored Gemstones

By Alethea Inns

In response to a challenging couple of years, 2022’s fashion trends are leaning toward colors that are bright, happy, and (frankly) delicious. Reminiscent of candy colors, the 2022 spring palette brings us back to childhood joy and simple indulgence—which also seems to be a perfect fit for this season’s jewelry. Much like how everyone has their favorite flavor, there is a colored gemstone for every taste, too.

Beyond the joy these gemstones evoke, there is a reason for how they get their color. For jewelers, understanding the scientific causes of color can add interest to a gemstone’s narrative and assist in building marketing campaigns. Further, knowing more about these mechanisms can also help in recognizing possible treatments and enhancements.

Color is the aspect of any object that may be described in terms of hue, tone (lightness), and saturation. In physics, color is associated specifically with the electromagnetic radiation of certain wavelengths of light visible to the human eye. In gemology, gemstones and diamonds absorb and transmit certain wavelengths, giving them the characteristic colors that we see with our eyes.

By Debbie Azar

As mentioned, there are three main elements that define color: hue, tone, and saturation. This is based on the three-dimensional color theory of the Munsell system, where hue (the color) circles a central axis; the saturation (richness of color) radiates from the centre outward; and the tone is on a vertical axis and determines the lightness and darkness of the color. (Fig.1)

Raspberry and Lime

To begin, let’s look at two gemstone favorites that resemble raspberry and lime candy: pink sapphire and Emerald cabochon (Fig. 2).

In Pantone’s Spring / Summer 2022 trend report, the fashion color authority identifies various pink and green hues. The luscious raspberry color ‘Innuendo’ from its New York Fashion Week Report and fun pinks, like ‘Bubblegum,’ from the London Fashion Week Report can be found in pink sapphires’ range of tones and saturations. Meanwhile, for greens, colors like the bright ‘Cascade’ and more subdued ‘Basil’ are also trending. The gemstone world has a wide variety of greens to choose from, including Emerald, as well as varieties of tourmaline, sapphire, and idiochromatic peridot (just to name a few).

Ruby/Pink Sapphire and Emerald are completely different minerals (Corundum and Beryl, respectively). Ruby is aluminum oxide (Al2O3), while Emerald is a beryllium aluminum cyclosilicate (Be3Al2Si6O18).

Not only are these completely different mineral groups, but they are on opposite ends of the color spectrum (Fig.3). Like raspberry and lime, they provide a contrasting balance that appeals to many tastes and preferences.

What pink sapphire/Ruby and Emerald have in common, however, is both are geologically defined by their color. Ruby is defined as the red variety of the mineral corundum—any corundum other than red is called ‘sapphire.’ Thus, a slightly pinkish ‘Ruby’ is not a Ruby, but, rather, a pink sapphire. Similarly, an Emerald is defined as the green to bluish-green variety of the mineral beryl—any beryl other than a saturated green is not an Emerald. A light yellowish green ‘Emerald’ is actually a green beryl. So, it is not enough that they are the right hue; they must also make the cut for saturation and tone (Fig. 4).

Chemistry Of Color

Some define the color of an Emerald based on its trace element chemistry. Indeed, Emeralds can be differentiated from green beryl based on the cause of its color, which is typically trace elements of chromium and/or vanadium. (Fig. 5)

Given, however, the constraints of the average jewelry professional and the level of difficulty in determining the chemical composition of a sample using standard equipment, the most common-sense definition of these gemstones is based on color (i.e. using Emerald’s strong-to-vivid saturation and medium-to-medium-dark tone of green to distinguish it from the light-green color of a Beryl).

Speaking of trace elements, though, these two gemstones have at least one bit of chemistry in common: the primary cause of color in both Ruby and Emerald is chromium (Cr)—specifically, the element in the Cr3+ electronic state (i.e. while chromium has 24 electrons, Cr3+ has three electrons removed from its outermost shell, giving it a net positive charge of three). So, how does Cr3+ cause Emeralds to be green and Ruby to be red? This occurs because the chromium absorbs light differently when it is in beryl (Emerald) versus when it is in Ruby or pink sapphire (corundum).

Trace Metals

Similarly, for many gemstones, the most common cause of color is the presence of a small amount of a transition metal ion. In solids (including gemstones), these metals occupy interstitial sites, such as tetrahedra or octahedra. They are typically iron, chromium, vanadium, manganese, cobalt, copper, nickel, or titanium.

These transition metal ions have an incomplete set of electrons in what are known as the 3D orbitals. For the sake of the ‘color conversation,’ the important thing to know is changes in the energy of these electrons correspond to the energy of visible light.

Perhaps you can see where this is headed: Cr3+ changes how the material interacts with light, and light equals color when in the visible spectrum.

As mentioned previously, Emerald is a beryllium aluminum cyclosilicate, while Ruby is an aluminum oxide—meaning, each gemstone, inherently, has a different molecular geometrical arrangement. In Emeralds, the Cr3+ is surrounded by six silicate ions, while the Ruby has six oxide ions. The beryl molecules interact with chromium weakly and the differences between the energy levels are reduced, producing green.

In corundum, if just one per cent of Al3+ ions are replaced with Cr3+ ions, the mineral becomes deep red in color and is known as Ruby (Al2O3:Cr3+). Meanwhile, in beryl, if one per cent of the Al3+ atoms are replaced by Cr3+, a green color is produced (Fig. 6).

Grape And Citrus

Given, however, the constraints of the average jewelry professional and the level of difficulty in determining the chemical composition of a sample using standard equipment, the most common-sense definition of these gemstones is based on color (i.e. using Emerald’s strong-to-vivid saturation and medium-to-medium-dark tone of green to distinguish it from the light-green color of a Beryl).

Speaking of trace elements, though, these two gemstones have at least one bit of chemistry in common: the primary cause of color in both Ruby and Emerald is chromium (Cr)—specifically, the element in the Cr3+ electronic state (i.e. while chromium has 24 electrons, Cr3+ has three electrons removed from its outermost shell, giving it a net positive charge of three). So, how does Cr3+ cause Emeralds to be green and Ruby to be red? This occurs because the chromium absorbs light differently when it is in beryl (Emerald) versus when it is in Ruby or pink sapphire (corundum).

The mineral Quartz is one part silicon and two parts oxygen (silicon dioxide [SiO2]). It is the most abundant mineral found on the Earth’s surface (12 per cent, to be exact).

While the cause of its color is not fully understood, scientists agree about one thing: much like corundum and beryl, the color of Amethyst and Citrine is thought to be caused by a transition metal: iron (Fe).

Whether Quartz is purple (Amethyst) or yellow (Citrine) depends on the valence state of the iron. The two different valence states in play are Fe3+ and Fe4+. Citrine’s color comes from the incorporation of very small Fe3+ aggregates in the crystal. Oxidizing the iron changes the color of the Quartz. To form the purple found in Amethyst, natural irradiation from the rocks surrounding the Quartz causes the Fe3+ atom to replace the silicon in the crystal lattice, which causes the formation of the FeO4 color centre as Fe4+. This absorbs wavelengths around 545 nm or 357 nm, which we see as purple.

Not every Quartz becomes an Amethyst. For this to happen, a good amount of iron (100 ppm) is needed, as well as enough radiation to cause the purple color.

Amethyst can be converted to Citrine with the application of heat. Likewise, Citrine changes to Amethyst through irradiation (Fig.7). Both changes can take place either naturally or through human intervention. Excessive heating, however, may change the distribution of the iron impurities at the different sites within the Quartz structure, making it impossible to convert Citrine back to Amethyst by subsequent irradiation.