The Science of Colour: How Irradiation and Heat Can Transform Quartz
Quartz is one of the most popular minerals worldwide, not only for its clarity and versatility but also for its wide range of colours. But did you know that some quartz colours are enhanced or even created through artificial treatments? By applying heat and irradiation (a type of energy exposure), scientists can bring out colours in quartz that might not appear naturally. Let’s dive into how these processes work!
Why Does Quartz Change Colour?
Quartz crystals can contain trace amounts of elements like iron and aluminium. On their own, these trace elements don’t usually produce visible colours. However, under certain conditions, such as exposure to heat or irradiation, the arrangement of these atoms changes in ways that produce colour.
Clear quartz / Rock crystal
For instance, in amethyst, iron impurities are present within the quartz crystal structure. Through a process called “charge transfer”, electrons in the iron atoms can shift positions, absorbing and reflecting different wavelengths of light. This shift can be induced by heat or irradiation and results in various shades. In the case of amethyst, the iron impurities interact with the crystal’s internal structure to produce its violet hue.
Aluminium impurities can also cause colour changes in quartz. When quartz contains aluminium in place of silicon within its lattice, it often requires a compensating ion to balance the crystal’s charge. These compensating ions can be sodium, potassium, or lithium. When irradiated, aluminium-bearing quartz can take on shades from smoky brown to nearly black. These colours are produced by the formation of “colour centres”, where specific sites in the crystal structure absorb light, creating the colours seen in smoky quartz and other varieties.
Heat Treatment: From Amethyst to Citrine, Prasiolite, and Neon Quartz
Heat treatment is a straightforward and widely used method to change the colour of quartz. Amethyst, known for its purple hue, can transform to a golden-yellow citrine, a soft green known as prasiolite, or even to a cloudy, lilac colour called neon quartz, depending on the temperature and conditions applied.
Amethyst to Citrine: When amethyst is heated to about 350–450°C, the purple colour fades, and the crystal turns yellow or orange, creating citrine. This colour change is due to a reconfiguration in the iron atoms, which releases electrons, shifting the wavelength of light that the crystal absorbs and reflects.
Amethyst to Prasiolite: At temperatures around 500°C, amethyst can shift from purple to green. The high temperature causes the iron atoms to further adjust within the crystal structure, resulting in the green tone known as prasiolite.
Neon Quartz: Heating amethyst even higher, to around 500–600°C, can result in a milky or cloudy appearance, sometimes referred to as “neon quartz”. This may very well also be the “lavender quartz” we often see in the market. At these temperatures, water trapped in the crystal structure is released as tiny droplets, creating a misty, translucent effect - also referred to as “turbidity” or being “turbid”.
These colours can appear naturally, however most of the commercial material on the market have been treated.
Prasiolite
Citrine (heated)
Blueberry quartz
Neon quartz
Irradiation: Creating Smoky Quartz, Lemon Quartz, and Morion
Irradiation is another method to alter the colour of quartz, involving exposure to gamma rays (high-energy radiation) that rearrange atomic structures within the crystal. This method is used both in nature, through exposure to natural radioactive sources, and in laboratories to produce colours not typically found in untreated quartz.
Clear Quartz to Smoky Quartz: Irradiation can turn clear or light-coloured quartz into smoky quartz, a variety known for its brown or grey tones. When quartz containing trace aluminium impurities is irradiated, it undergoes a charge change, creating “colour centres” where light is absorbed. These colour centres, formed around the aluminium ions, are responsible for the smoky tones. The depth of the colour depends on both the level of aluminium impurities and the irradiation dose.
Smoky Quartz to Morion: If irradiation continues, smoky quartz can darken further to an opaque black variety called morion. Morion is a very dark form of smoky quartz and is often highly sought after in the gem trade for its dramatic appearance.
Lemon Quartz: Lemon quartz, also known as “green gold”, is a bright, chartreuse quartz variety produced by irradiating quartz containing both aluminium and lithium impurities. The yellow-green colour of lemon quartz is typically stable but requires precise conditions, and natural deposits are rare. In the lab, this colour is created by irradiating quartz with a high lithium content, which produces a unique chartreuse hue with a hint of green.
These irradiation-induced colours are generally stable, but like heat-treated quartz, they result from human intervention rather than natural geological processes.
Smoky quartz
Morion
Lemon quartz
Is Treated Quartz Still Quartz?
Absolutely! Treatments like heat and irradiation change the crystal’s colour but don’t alter its overall structure or composition. These processes simply enhance or reveal colours that may not appear naturally. It’s an interesting way to bring out the potential that quartz holds within its crystal lattice.
Summary of Colour Changes in Quartz
The quartz varieites have been summarised below from an article from the Journal of Gemmology 2012. The journal article can be found in the references section below.
| Quartz Type | Colour | Mechanism/Cause of Colour | Treatment/Temperature | Process |
|---|---|---|---|---|
| Amethyst | Violet | Fe4+ and Fe2+ centres, electron transitions | Stable up to 350-450°C | Natural quartz with iron, irradiated and cooled; loses colour when heated above 450°C |
| Citrine | Yellow to Brown | Fe3+ centres, formation of hematite particles | Heat treatment (350-450°C) | Heat-treated amethyst; Fe3+ changes to hematite particles giving yellow to brown hue |
| Prasiolite | Green | Fe3+ to Fe2+ transformation | Heat treatment (400-500°C) | Heated amethyst with iron centres changing to Fe2+, resulting in a green colour |
| Blueberry Quartz | Violet to Dark Blue | Fe4+ and Fe2+ centres | Gamma irradiation + heat | Irradiated and heated prasiolite; Fe4+ and Fe2+ produce violet-blue colour |
| Bleached Quartz | Milky White | Water droplet formation within structure | Heat treatment (>500-600°C) | High heat expels water as droplets, scattering light and bleaching violet colour |
| Bi-Coloured Quartz | Amethyst and Citrine zones | Fe4+ in rhombohedral sectors (amethyst); Fe2O3 in other sectors (citrine) | Heat treatment (475°C) | Colour-zoned quartz; violet amethyst sectors turn yellow with heating, reversible |
| Smoky Quartz | Brown to Black | Al3+ centres with alkali ions and electron-hole centres | Gamma irradiation | Al-containing quartz irradiated to produce electron-hole centres for brown/black hue |
| Lemon Quartz | Yellow-Green (Chartreuse) | Al3+ centres with Li or hydronium ions | Irradiation + heat treatment | Lithium-rich quartz irradiated, forming greenish-yellow centres |
| Green (Wet) Quartz | Green | Interaction of gamma radiation with water | Gamma irradiation | Water-rich quartz irradiated; water molecules split, creating green colour centres |
References
The Journal of Gemmology (2012). Review of Some Current Coloured Quartz Varieties. The Journal of Gemmology, 33(1–4), 32–42
Cohen, A.J. (1985). Amethyst color in quartz: the result of radiation protection involving iron. American Mineralogist, 70, 1180–5.
Lehmann, G., & Bambauer, H.U. (1973). Quarzkristalle und ihre Farben. Angewandte Chemie, 85, 281–9.
Rossman, G.R. (1994). Colored varieties of the silica minerals. Reviews in Mineralogy and Geochemistry, 29, 433–63.