The UV–Vis spectra of amethysts had a broad absorption bands at 545 nm and 345 nm. Therefore, the samples heated at different temperatures were tested by UV–Vis spectroscopy. The absorption bands at different wavelengths of amethyst UV–Vis spectra correspond to the relevant color centers 12, 16, 37. The diameter of the “water bubbles” is from 20 to 100 nm, which can cause Rayleigh scattering of light 36. It can be related to the destruction of the S 1 centers () and I centers (Fe 3+ in an interstitial site) 34 and the aggregation of water to form “water bubbles”. When the temperature is above 600 ☌ (higher than the phase transition temperature of amethyst), the amethyst becomes milky and turns lighter. Chroma (C*) and hue angle (h°) can be calculated by a* and b* as following: The CIE 1976 L*a*b* uniform color space system is composed of colorimetric coordinates a*, b* and lightness (L*). However, there are few studies on the color evaluation and analysis in the process of heat treatment of amethyst. Then the International Commission on Illumination (CIE) 1976 L*a*b* uniform color space system has also been gradually applied to the mineral color evaluation and grading system, such as sapphire 19, alexandrite 20, 21, color change garnet 22, 23, peridot 24, 25, tourmaline 26, 27, citrine 28 and jadeite 29, 30, 31. In this method, the colored diamonds were graded in standard illumination box and a 6504 K fluorescent lamp was used as lighting source. In order to obtain and analyze the color of minerals, the color grading method of colored diamond was proposed in 1994 18. The main absorption bands of amethyst in UV–Vis spectra are located at 950 nm, 540 nm and 360 nm, which is considered to be related to the transition of Fe 4+ in tetrahedral coordination 9, 12, 16, 17, and these absorption bands show different degrees of changes after irradiation or heating. The study of UV–visible spectroscopy of amethyst also provides support for the study of the existing form of iron element in amethyst.

The color change of amethyst after heating and irradiation is related to the “color center activated by iron” 7, 15. Amethyst could become citrine when the temperature is over 500 ☌ 10, and the violet and green color could be appeared by irradiating the colorless quartz 14. Many scholars have studied the enhancement methods of amethyst to discuss the mechanism of the coloration 7, 13, 14. In addition, holes created during quartz irradiation can also act as charge compensators 12. It was postulated that amethyst color can be related to the replacing Si 4+ with Fe 3+, and alkali metal ions (Li +, Na + or H +) are introduced to maintain the charge balance 11. The colors of amethyst 6, 7, citrine 8 and prasiolite 7, 9, 10 are all related to iron. When the quartz contains impurities such as iron, aluminum and titanium, it will make quartz appear rich in color 5. It is widely distributed in the crust and the origins are all over the world, such as Brazil, Uruguay, Canada, Sri Lanka 1, Rwanda 2, Morocco 3 and Arizona 4, etc. A more appropriate heating temperature to obtain citrine by heating amethyst is about 560 ☌.Īmethyst is a kind of α-quartz with beautiful violet color. When the initial color is darker, the color difference of heated amethyst is larger, and the easier it is to change the color after heat treatment. The color change degree of heated amethyst is related to its initial color. The color at different temperatures can be divided into three stages: The amethyst stage with temperature below 420 ☌, the prasiolite stage with temperature between 420 and 440 ☌ where the color center is the most unstable, the citrine stage with temperature above 440 ☌. The absorption band at 545 nm in the UV–visible spectrum can be related to a charge-transfer transition of Fe 3+ and O 2−, which has a significant relationship with amethyst lightness and chroma. The results show that the amethyst color has no significant relationship with cell parameters but the crystallinity index decreases as temperature rises.

The effect of heat treatment on amethyst color was studied from a new perspective of chromaticity of gemstones and the cause of amethyst coloration was discussed based on the results of X-ray diffraction, ultraviolet–visible spectroscopy.

Amethyst quartz