Buy Genuine Alexandrite Stone, the Gem with a 100% Green to Red Color Shift

Alexandrites are remarkable and rare gemstones. They display an extraordinary colour change according to the ambient lighting, from emerald green in daylight to ruby red in incandescent light from tungsten lamps or candles. While this colour change has been correctly attributed to chromium impurities and their absorption band in the yellow region of the visible light spectrum, no adequate explanation of the mechanism has been given. Here, the alexandrite effect is fully explained by considering the von Kries model of the human colour constancy mechanism. This implies that our colour constancy mechanism is real (objective) and primarily attuned to correct for the colour temperature of black-body illuminants.

Alexandrites are rare and highly-prized gemstones, which were first discovered around 1830 in Russia, and named after the future Czar, Alexander II, and later found in other countries such as Brazil and Sri Lanka1. They are prized for their dramatic colour change under different illumination: that is ruby-red under candlelight or incandescent lamplight, and emerald-green under natural daylight1. This is attributed to the Cr3+ impurities in the BeAl2O4 atomic structure which have strong optical absorption centred at the wavelength of yellow light. While the stones scatter blue, red and green light in proportions which vary with the illuminant2,3,4, this is not sufficient to explain the colour change in alexandrites as it is as true of any coloured object as it is of alexandrites. Here we give a complete explanation of the alexandrite effect. Taking into account the responses of the cone photoreceptors in the human eye, we show that the ratio of green to red stimuli under the different illuminants changes much more for an alexandrite than it does for normal coloured objects, which have broad absorption bands (inks, pigments, paints, fruit and flowers, etc). This overrides the mechanism by which the human visual system corrects for illuminance and helps understand the working of this mechanism for colour constancy. We expect these findings to have implications for theories of the colour perception of the human visual system in the somewhat disparate fields of traditional colour science5, the science of optical illusions6, and the study of individual perceptions, as in Impressionist paintings7 or the dress that went viral8.

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An alexandrite changing colour. The Russian alexandrite BM41178 from the collection of the Natural History Museum, London was photographed in (a) daylight and (b) incandescent light with the camera colour-correction feature switched off. The stone was placed on the matching colour on a printout of the CIE 1976 colour chart. A piece of white paper was included in the pictures, under the stone in (a) and nearby (top left) in (b). The corresponding images after colour balancing to make the paper white are shown in (c,d).

A wide variety of physical phenomena from interference and diffraction to spectrally selective absorption can give rise to dramatic colour effects such as iridescence10. Explanations of the colour change in alexandrites have been given which correctly invoke the spectrally-selective Cr3+ absorption band in the yellow region of the spectrum1,2,3. Liu et al.2 took their explanation further, invoking also colour constancy. Experimentally, they said, the human visual system corrects for hue angle changes only up to 20 under different illuminations (hue angle is explained in the Supplementary 2). Calculating the change in hue angle for alexandrites and finding it to be greater than 20, they attributed the absence of colour constancy in the alexandrite effect to that. However, the blue-to-brown hue angle change of the white paper in Fig. 1a,b is about 180, yet colour constancy occurs. Recently a similar analysis of a weak alexandrite effect in purple flowers and low-quality alexandrite stones has been reported11.

The human retina has three kinds of colour photoreceptors, or cones. The S cones detect short-wavelength light (blue), the M, medium wavelengths (green) and the L, long wavelengths (red). See Supplementary 4. In Fig. 2, the spectra of the responses of the cones, the illuminants and the transmittance of the stones (SI) are combined, in order to find out what is perceived. Figure 2 shows the spectral responses of the L, M and S cones14, with, Fig. 2a, the standard daylight spectrum D6514, and Fig. 2b, the candlelight spectrum (black body with a colour temperature of 1850K, which is approximately the standard illuminant A14). In Fig. 2c,d, the products of the illuminant spectra and the L, M and S spectra are plotted. The integrals of these curves correspond to the signals sent by the cones, and their values (normalised so the largest is 1) are marked. These LMS values are converted using an LMS-RGB conversion matrix (see Supplementary 5, Eq. S1) to the RGB colours used in the fill. These are approximately the colours that a white object would be perceived to have if we did not have colour constancy; they are also approximately the colours that an uncorrected camera records, as in Fig. 1a,b. In Fig. 2e,f, the wavelengths absorbed by the stones are removed from the illuminant spectra; what remains are the spectra of the light scattered by the stones. By daylight, the green (and some blue) dominates and the red is weaker; conversion to RGB gives the green used as fill in Fig. 2e. By candlelight the red dominates (Fig. 2f). Now we apply colour constancy, using a standard model of the mechanism, the von Kries correction15. See Supplementary 6 for details of our calculations and Supplementary 7 for a discussion of more advanced models16,17. The correction makes the colours of Fig. 2c,d white. Applied to the data of Fig. 2e,f, we get the data of Fig. 2g,h, where conversion to RGB again gives the fill colours.

Mapping of the alexandrite effect. In (a), the colours calculated as for Fig. 2 for daylight (D65) and incandescent light (BB1850K) are plotted as a function of absorption peak position and peak width as background and overlaying spots, respectively. The small region where the full alexandrite effect occurs is outlined. The alexandrite stones R1 and R2 are marked on the map and fall within the outlined region. In (b) the map is calculated as for (a) but with the blue absorption band removed.

The alexandrite stones (BM41177 and BM41178, both from Russia) were from the collection of the Natural History Museum, London, UK. They were observed and photographed under natural daylight (north light, in the shadow of a large building on a summer day with sun and scattered clouds) and in a darkroom with only incandescent light for illumination. Candlelight and a tungsten lamp were both used with very similar results; the images in Fig. 1b,d were made with the tungsten lamp. The stones were placed on a printout of a CIE 1976 UCS colour chart9 as a background, and a piece of white paper was placed in the picture. The position was selected by five judges (CR, AH, EDT, AJD and DJD for one session and AH, RH, EDT, AJD and DJD for another session). Each judge moved the stone as they thought best to improve the colour match to the chart; this process converged each time on a position at which no-one proposed a further move, i.e. a position to which all judges consented. A digital camera (Nikon D5100, 16 MP CMOS detector) was used with all colour-balance and selective-colour software switched off, and flash and autofocus also off. Images were exported from the camera as NEF files (raw 14-bit data from the image sensor) and converted to JPGs by opening them in Microsoft Photos and saving a JPG copy.

Colour balancing was applied to create the corrected images (Fig. 1c,d). For Fig. 1, we used a simple approximate technique for RGB image files. These photos were corrected by dividing all pixel (R, G, B) values by the (R, G, B) values of the white paper. Then the image was re-created using image = Image[data]. This makes the paper RGB-white, (1, 1, 1), in the colour space of the camera and of any RGB or CMYK display unit or printer (see Supplementary 1). The procedure is not very accurate for other colours because the response curves of the R, G and B photodetectors in the camera are not known nor used, and nor are the spectra of the R, G and B light sources in a computer monitor or the corresponding spectra of the inks in a colour printer. In particular, the blue cast of the red CIE chart in Fig. 1d around the stone, where the light was brightest, appears to arise from a sub-linearity of the photodetectors at high light levels (see Supplementary 1).

Hi!We are writing an educational article on color temperature and the color of alexandrite for our upcoming coloured stone web-portal. Can I use your fantastic images if I give you due credits and link the blog?

hi claire , if your alexandrite can apear BLACK ! , so its mean you have genuin one , your alexandrite is a natural stone , because the synthetic alexandrite do not apear emerald green or BLACK , so take close and look your stone , its can be very valuable

While pieces that have been enhanced are not necessarily any less beautiful or genuine, most elite jewelers will only work with naturally occurring gemstones. Besides their rarity, natural gemstones are often prized for being part of our geological heritage here on this planet.

Alexandrite that is lab-created, for all intents and purposes, is chemically identical to its naturally occurring sibling. However, that being said, lab-grown alexandrite mimics each other, while genuine pieces from mines are each unique by the very merit of their formation process.

Many rare fine jewels are being made in laboratories partly due to ethical concerns in certain mines. If you are investing in a genuine piece of gemstone jewelry, it is paramount to go to a trusted dealer that only works with ethically sourced mines where people work in fair and safe conditions. 2ff7e9595c