Color Change Garnets: Gemstone Information
Color Change Garnets arises because stones are able to transmit two different ranges of wavelengths of light between areas of absorption. Depending on the light source, which may be enhanced in certain wavelengths, one of the ranges will be dominant. Thus when viewed in daylight, which is enhanced in the short wavelength part of the spectrum, the stone will be either violet, blue or green or some combination of these colours, and in incandescent light, which is enriched in long wavelengths, either red or orange or a combination of these. The elements largely responsible for this ‘alexandrite effect’ are chromium and vanadium generally in conjunction with manganese. Colour change behaviour has been correlated with vanadium, chromium, manganese, magnesium and iron components (see also optical effects under individual species name).
Colour change garnets have been reported from Madagascar (Ilakaka and Bekily), Tanzania (Umba Valley and Tunduru) and Sri Lanka (Athiliwewa and Embilipitiya). Also recorded from East Africa and Russia.
Colour change garnets fall into two distinct groups. The most common are those of the pyrope-spessartine series where extensive substitution of magnesium by manganese occurs. These also contain traces of chromium and/or vanadium up to 2 wt%. Less common are the chromium-rich pyrope with Cr3+ > 3 wt%. Colour change has also
been reported for pyrope-almandine, spessartine-grossular-almandine, spessartine-grossular-pyrope and andradite. Thus physical and chemical properties can vary appreciably.
Studies of the East African and Madagascar material above gave RIs between 1.773 and 1.763 and between >1.81 and 1.748 respectively with decreasing spessartine component; likewise SGs varied between 3.98 and 3.89 and between 4.1 and 3.784.
Manson and Stockton recorded that stones from Madagascar exhibited, with increasing iron content, colour changes from greenish grey (daylight) to pink or red (incandescent) and greenish blue (daylight) to reddish violet
Colour change is highly variable in the breadth of hues, tones and tints. These depend on the degree of molecule admixture and trace element impurities.