The quartz group is home to a diverse range of minerals including the crystalline varieties of amethyst, citrine and rose quartz, which are prized for their attractive hues, plentiful sources and relative affordability compared to other similarly-coloured gemstones.
Quartz is an allochromatic mineral that owes its colour to trace impurities rather than its own constituent elements. Rock crystal or ‘clear quartz’ is colourless and the most chemically pure form of quartz, which is silicon dioxide. Other varieties are coloured by different impurities in the crystal structure. These trace impurities resonate in light and absorb specific parts of the spectrum. The remaining light transmitted or reflected to the eye gives colour to the gemstone.
Amethyst, the purple variety of quartz, ranges from the palest of lavender hues through to a deep vivid violet, with the finest colour being a strongly saturated bluish-purple with red flashes – typical of top quality Brazilian, Zambian and Uruguayan stones. Amethyst often displays colour banding or zoning, sometimes seen as a chevron ‘zig-zag’ pattern, due to an internal twinned structure.
Whilst the actual cause of colour in amethyst is still uncertain, a popular theory suggests that the presence of iron within the crystal lattice, paired with irradiation from surrounding rock masses, is responsible.
Citrine, the golden yellow variety of crystalline quartz, has colours ranging from a near- colourless pale yellow, to deep orange-yellow and brownish-yellow. Naturally occurring citrine is rare, with the majority of stones on the market resulting from the heat treatment of amethyst, or sometimes heat-treated smoky (brown) quartz. This is a generally accepted treatment as the process does not add anything to the stone and only mimics what can occur in nature.
Whilst most amethyst is heated to produce citrine, a very small number of amethyst deposits contain material that turns a yellow-green to green colour, turning it into prasiolite. This pale green mineral can form by natural heating processes, but most available material on the market is treated amethyst and incorrectly named ‘green amethyst’.
Rose quartz is named after its delicate pink hue. The soft translucence of this stone is due to tiny inclusions, giving it a cloudy appearance. Colours range from pale whitish-pink, purplish-pink through to deep pink, said to be caused by traces of either titanium oxide or manganese. Stones with a clear transparency and deep colour are considered fine quality and are often faceted.
Madagascar reigns supreme as a source for high-grade rose quartz, with deep pink tones and extraordinary ‘star rose quartz’ specimens. These ‘star’ stones exhibit an asterism – a six-pointed star – caused by fine needles of rutile arranged in parallel layers, oriented at 120 degrees to one another. As incident light reflects off the inclusions, a bright star appears to glow on the surface!
Coloured quartz is popular and plentiful, making these playful stones ideal for carvings, large centrepiece gems and in gradient colour settings for statement jewellery designs.