Rio Grande citrine is a trade name for citrine produced by heating amethyst from the Rio Grande do Sul state of southern Brazil. The treatment converts the purple body colour of the parent amethyst to the golden yellow, golden orange, and at the saturated end the slightly reddish-orange tones recognised in the citrine market. Almost all citrine in the modern jewellery trade is heat-treated material of this type rather than the rare and seldom-encountered natural unheated citrine.

Source and treatment

The amethyst geodes of Rio Grande do Sul, particularly from the Ametista do Sul and Salto do Jacuí districts, have been the world's principal source of commercial amethyst since the 1940s. The basaltic lava flows of the Paraná Basin host enormous geodes, some weighing several tonnes, lined with amethyst crystals that vary considerably in colour saturation. Lower-saturation amethyst — pale to medium purple material that does not command top prices as amethyst — is the principal feedstock for citrine production.

The treatment is straightforward: amethyst is heated to between 400 and 500 degrees Celsius in a controlled-atmosphere kiln. At these temperatures, the iron impurity centres responsible for the purple colour shift their oxidation state from the Fe⁴⁺ form that produces the violet absorption to the Fe³⁺ state that produces yellow-to-orange. The colour change is permanent under normal storage and wear conditions and is not reversible without re-irradiation followed by particular thermal handling.

Disclosure and detection

Heat treatment of amethyst to citrine is universally accepted in the trade and is a routine, undisclosed practice in most commercial supply. The AGTA and CIBJO position the treatment under their standard heat-only categories, and laboratory reports describe the colour as treated where the question is asked, but the treatment is not disclosed at point of sale in the same explicit terms required for, say, beryllium-diffused sapphire.

Distinguishing heat-treated Rio Grande citrine from natural unheated citrine is difficult to impossible without provenance documentation. The most reliable indicators are colour distribution — natural citrine is more often pale lemon-yellow with a slightly smoky undertone, while heat-treated material tends to a warmer orange — and the rare presence of distinctive growth-feature inclusions in unheated material. For practical trade purposes, citrine is treated as treated unless explicitly documented otherwise.

Colour and grading

The Rio Grande citrine market grades along the standard quartz-family axes of hue, tone, and saturation. The most desirable colour is variously described as Madeira citrine for the deep reddish-orange tones, palmeira citrine for the brighter golden-orange middle tier, and rio grande citrine for the standard golden-yellow material. Trade naming varies between dealers and is not standardised across all markets.

Crystal sizes from Rio Grande are large by the standards of most coloured-stone production. Faceted citrines of fifty carats and above are routine, with show-quality stones of several hundred carats not unusual. The combination of saturated colour, large size, and modest pricing makes Rio Grande citrine one of the most accessible coloured-stone categories at the larger sizes.

Care

Citrine is a quartz with hardness 7 and good toughness, suitable for ring-stone use with reasonable care. Avoid prolonged exposure to high heat, which can drive further colour shift, and avoid impact on facet edges where the brittle conchoidal fracture of quartz is exposed. Cleansing with mild soap and warm water is appropriate; ultrasonic and steam cleaning are generally safe but should be avoided on stones with visible inclusions.

In the trade

For the working trade, Rio Grande citrine occupies the role of a reliable, available, large-size warm-toned coloured stone. Its November birthstone status drives consistent retail demand, and the wide colour range from lemon to deep madeira allows considerable design flexibility. The treatment is universal and accepted, and the buyer's main consideration is colour preference rather than provenance.

Further reading

• GIA — Citrine reference
• GIA Gems & Gemology — quartz treatment literature
• AGTA — coloured-stone treatment terminology