Colour change is the optical phenomenon in which a single gemstone shows distinctly different hues when viewed under different light sources, typically daylight or its equivalent (around 6500 K) on the one hand, and incandescent light or its equivalent (around 2856 K, also referred to as Standard Illuminant A) on the other. The phenomenon is caused by selective absorption of certain wavelengths of light by the stone's chromophore ions, with the result that the stone transmits a balance of remaining wavelengths that the eye reads as one colour under one illuminant and another colour under the other. The most celebrated colour-change stone is alexandrite, a chromium-bearing variety of chrysoberyl, but the phenomenon is also seen in certain garnets, sapphires, fluorites, spinels, and a small number of other species.

The physical basis

The mechanism, sometimes called the alexandrite effect after the most famous example, depends on the absorption spectrum of the stone showing two transmission windows: one in the blue-green region and one in the red. Daylight is rich in blue-green wavelengths, so a stone with this absorption profile transmits predominantly blue-green light in daylight and appears green or blue-green. Incandescent light is rich in red and yellow wavelengths and poor in blue-green, so the same stone transmits predominantly red light and appears red or purplish-red. The eye, lacking absolute reference points and adapting to the illuminant, reads each transmitted balance as a different colour. Chromium is the principal chromophore in alexandrite; vanadium plays a role in some colour-change garnets and sapphires.

Alexandrite

Alexandrite, named for the future Tsar Alexander II of Russia after its discovery in the Ural emerald mines in 1830, was the first stone in which the phenomenon was systematically described and remains the gemmological reference. Russian alexandrite shows a strong colour change from blue-green in daylight to purplish-red in incandescent light, and in the finest stones the change is described as "emerald by day, ruby by night". The Russian Imperial colours were green and red, which gave the stone immediate national resonance and contributed to its high status. Modern alexandrite comes from Brazil (notably Hematita, Minas Gerais), Sri Lanka, India, Tanzania, and a small Madagascar production; the strength of the colour change varies with locality and with the quality of the rough.

Other colour-change stones

Colour-change garnets are a recognised category, principally pyrope-spessartine garnets from Tanzania, Madagascar, and the East African belt, showing changes from blue-green or grey-green in daylight to purple or pinkish-red in incandescent. The mechanism involves chromium and vanadium chromophores. Colour-change sapphire, also chromium and vanadium-bearing, occurs in Sri Lankan, Tanzanian, and certain Thai and Cambodian deposits, with daylight blue or violet shifting to purple or red in incandescent. Colour-change fluorite, colour-change spinel, and a number of other species can show the phenomenon at varying intensities.

Grading and disclosure

Major laboratories grade colour change on a scale that includes strong, moderate, and weak categories, with strong-change stones commanding substantial premiums. The Gübelin Gem Lab, AGL, GIA, and SSEF all describe colour-change phenomena on their reports for stones in which the effect is significant. The percentage of colour change - the visual difference between the two colours - is typically estimated by the laboratory and may be expressed as a numerical figure or as a verbal description. The strongest changes, comparable to fine Russian alexandrite, are extremely rare; most colour-change material on the market shows partial or weak change, and the buyer's task is to assess the strength of the effect and the attractiveness of both end-state colours, since a stone that goes from a muddy green to a muddy red is no improvement on a stone that simply has one good colour.

Usage in the trade

Colour change is a marketed phenomenon, and the trade often demonstrates the effect under controlled lighting at the point of sale. The buyer should examine the stone under daylight (north-facing window or daylight-equivalent LED), incandescent (a 60 to 100 watt incandescent bulb or its halogen equivalent), and ideally under fluorescent and modern LED lighting, since modern domestic illumination has shifted away from pure incandescent and the stone's appearance under typical wear conditions matters. A strong colour change in the laboratory under controlled illuminants does not always translate to a dramatic effect under modern domestic lighting.