Unveiling the science behind
Gemstone colours
Ever wondered why gemstones are the colour they are? Colour is one of the most captivating and defining characteristics of gemstones. We often choose our jewellery, clothes, accessories and paintings in our favourite colour. Colour has the power to affect
our moods and is omnipresent in our daily lives. Did you know that colour is a product of complex interactions between light and matter, and in the case of a gemstone, the gemstone’s chemical composition and its unique crystal structure.
By Breanne Avender and Archana B. Jaswani
ALLOCHROMATIC AND IDIOCHROMATIC
To understand how colour forms in gemstones, it is essential to explore the physical, chemical, and optical properties that contribute to this phenomenon. Gemstones fall into two categories when it comes to how they achieve their colour: allochromatic and idiochromatic.
Another name for allochromatic stones is “other-coloured.” Initially clear by nature, they acquire colour as they form in the Earth. The colouring elements – also known as transition elements, trace elements, or impurities0 – do not form part of their primary chemical composition. These transition elements include titanium, copper, manganese, vanadium, iron, cobalt, nickel, and chromium. Each of these elements, or sometimes a combination of them, produces the colours we see in gemstones.
These elements absorb specific wavelengths of light, giving each gemstone its unique colour. For example, chromium is responsible for the red colour in rubies. When chromium ions are present in the crystal structure of corundum (the mineral family to which rubies and sapphires belong), they absorb certain wavelengths of light, leaving behind the red appearance.
Iron and titanium are responsible for the blue colour in sapphires, absorbing yellow and red light so that blue wavelengths dominate. Copper can create green or blue hues in gemstones such as turquoise and Paraíba tourmaline. Vanadium produces the green colour in emeralds, while manganese contributes to the pink colour in sapphires. Idiochromatic stones, by contrast, are self-coloured. Their colour is an inherent part of their chemical make-up. For example, turquoise’s chemical formula is CuAl₆(PO₄)₄(OH)₈·4H₂O, where copper is both a key component and the colouring element.
Iron causes the bright green of peridot and the rusty red of almandine garnet. Malachite is also coloured by copper, while spessartine garnet’s rich orange hue comes from manganese. Most gemstones are allochromatic, while only a few are idiochromatic.
Photo © Shinobi/courtesy Bigstockphoto.com
Photo courtesy AI-generated image (ChatGPT/DALL-E)
LIGHT AS AN INFLUENCE
Colour in gemstones can also be attributed to how they interact with light. Light is made up of electromagnetic waves, and when it strikes a gemstone, it can be absorbed, transmitted, or reflected. The way these interactions occur depends on the gemstone’s composition and internal structure, which are unique to each mineral.
When white light, a combination of all visible colours, hits a gemstone, the material absorbs certain wavelengths while allowing others to pass through. The wavelengths that are absorbed depend on the energy levels of the atoms or ions within the gemstone. The wavelengths that are not absorbed are either transmitted through the stone or reflected back to the observer’s eye, creating the gemstone’s visible colour. In simple terms, the colour we see is the light that is not absorbed.
If a stone absorbs all light, it appears black. If all wavelengths pass through, it appears colourless, like a diamond. Some gemstones, however, bend these general rules. Alexandrite is a well-known example, displaying a colour change between red and green. Due to its unique crystal structure, it balances between these two colours. In daylight, which contains more blue-green light, it appears green; under incandescent light, which is richer in red wavelengths, it appears red.
Beyond chemical composition, the arrangement of atoms in the crystal lattice, the three-dimensional structure of the gemstone, also influences how light interacts with the material. This structure determines how light is absorbed and reflected at different wavelengths.
Gemstones with a highly ordered crystal structure may allow light to pass through more efficiently, resulting in clearer and more vibrant colour. In some cases, the interaction of light with the crystal lattice produces optical effects such as pleochroism, where a gemstone…
exhibits different colours when viewed from different angles. This effect occurs because the gemstone’s crystal structure absorbs light differently depending on its direction of entry.
SCATTERING LIGHT
An Alexandrite ring. Photo © Boykung/courtesy Bigstockphoto.com
Light scattering can cause colour variations and change our perception of colour.
In some cases, the internal structure of a gemstone, such as the presence of inclusions or microscopic defects, can scatter light and influence how colour is perceived. For example, while diamond is known for its remarkable brilliance and dispersion due to its crystal structure, numerous minute cloud inclusions can cause it to appear milky rather than colourless and brilliant.
Certain gemstones may also display colour zoning, where colour varies across the surface or within the body of the stone. This occurs when the gemstone’s crystal grows under changing environmental conditions, leading to varying concentrations of trace elements.
For instance, tourmaline can exhibit a range of colours
from green at one end to pink or red at the other, depending on the concentration of elements like iron, chromium, and manganese.
SCATTERING LIGHT
Did you know that treatments can also alter the colour of a gemstone, giving it the hues we admire?
Gemstones can undergo various treatments to enhance or modify their colour. Heat treatment is commonly used to intensify the colour of sapphires and rubies. By heating the gemstone to high temperatures, impurities or trace elements become more stable, altering the absorption of light and enhancing the stone’s colour. For example, heat treatment can transform a pale-yellow sapphire into a more vivid blue.
Similarly, irradiation uses high-energy radiation to alter the colour of gemstones, typically producing shades of green, blue, or purple. Understanding the science behind gemstone colour allows us to better appreciate the natural processes that create these dazzling hues and the rich diversity found in the world of gemstones. Next time you admire a stunning sapphire in a display window, take a moment to appreciate it not only for its beauty, but for the remarkable journey and natural processes that created its colour.
First published by www.jewellerybusiness.com, 28 January 2026.