By Yaĝé Enigmus
The final member of the ugrandite garnet series is the mineral andradite garnet. One of the most valuable gem garnet species, andradite garnet is prized among mineral collectors and gemstone enthusiasts alike.
Andradite garnet is the calcium iron nesosilicate member of the garnet group and perhaps the most lustrous when compared to other garnet minerals. This garnet mineral was first documented in Norway by Western scientists in the year 1868, and was named after José Bonifácio de Andrada e Silva, a Brazilian poet, naturalist, and statesman. Andradite garnet often exhibits an earth brown colouration, which is derived from the ferric iron (Fe3+) that makes up part of its pure chemical structure.
A cluster of andradite garnet crystals from the Kayes Region of Mali; Image: Mindat/ Joseph A. Freilich
The most sought after variety of andradite garnet is known as “demantoid garnet”, which ranges in colour from yellow-green to emerald green. First documented by Western scientists after it was encountered by miners in the Ural mountains of Russia in 1868, it was this variety of andradite which revealed the mineral’s superior lustre and dispersion, surpassing that of the diamond. The name “demantoid” is derived from the French word for diamond, ‘diamant”, in reference to the stone’s diamond-like optical properties. The shine of demantoid garnet has made it one of the most valuable of all garnet gemstones and earned it the nickname “the star of garnets”. The green colour of demantoid garnet is caused by impurities of trivalent chromium (Cr3+), with the ferric iron (Fe3+) in its pure structure lending yellow tones to some crystals. A significant quantity of demantoid garnet was discovered in the African country of Namibia in the late 1990s, which brought more gem-grade material to market. This was followed by the discovery of another significant deposit in northern Madagascar just over a decade later, although small amounts of high-quality material have also been found in Italy, Iran, Afghanistan, and Pakistan.
An emerald green demantoid garnet crystal in matrix of asbestos from Lombardy, Italy. Image: Mindat/ Chinellato Matteo
Even rarer than demantoid garnet is the yellow variety of andradite garnet, which is known as “topazolite garnet” in reference to the golden colour sometimes exhibited by topaz. Topazolite garnet may range in colour from bright yellow to a deep golden honey colour. Topazolite's hue is primarily caused by ferrous iron (Fe2+) but it is thought that this iron may take part in a charge transfer with titanium (Fe2+-Ti4) that also contributes to topazolite’s appearance in some cases. Since topazolite garnet is significantly rarer than demantoid garnet, some yellow-green andradite garnet gemstones are labelled as topazolite, but true topazolite garnet is devoid of any green hues.
A cluster of topazolite garnet crystals from Kerman Province, Iran; Image: Mindat/ Vachik Hairapetian
Black andradite garnet is one of the most abundant varieties of the mineral, and it is known by the name “melanite garnet”. The colour of melanite garnet is primarily due to the presence of quadrivalent titanium (Ti4+) which takes part in charge transfers with either ferrous iron (Fe2+-Ti4+) and/or divalent manganese (Mn2+-Ti4+), earning this andradite variety the nickname “titanium andradite”, although charge transfers between ferrous and ferric iron (Fe2+-Fe3+) are also thought to contribute to melanite garnet’s colour in some cases. Despite being the only gem garnet variety that always occurs as opaque crystals, melanite garnet is still valued as a faceted gemstone due to its ideal lustre; melanite may serve as an unusual and affordable substitute for lustrous black stones such as black spinel and black diamond, possessing the ability to mimic these stones optically while physically functioning in a manner similar to softer stones such as black tourmaline and onyx.
The least well-known variety of andradite garnet is one that occurs with an otherworldly iridescence. This multi-coloured variety of andradite is affectionately referred to as “rainbow garnet” or “rainbow andradite”. Garnet crystals which display iridescence were first documented in the late 1990’s, but such stones did not gain widespread recognition until 2004 when a deposit of highly iridescent andradite garnet was discovered in the Nara Prefecture of Japan. While the body colour of this andradite garnet variety is typically identical to that of brown andradite, it is unique in that it displays an iridescent sheen which is observable both on the surface of individual crystals and in many cases within the crystals themselves. The spectrum of colours seen in these garnets is thought to be produced by thin film interference caused by their unique internal structure, which is composed of lamella that alternate between pure andradite garnet and andradite containing minute impurities of trivalent aluminum (Al3+). When used as cabochons and/or faceted gemstones, rainbow garnets are often given freeform cuts due to the need to preserve as much crystal structure as possible in order to avoid removing sections of the gem which contain the lamellar formations that produce the desired iridescence.
A cluster of rainbow garnet crystals from Nara Prefecture, Japan; Image: Mindat/ Jingnan Zhang
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© Yaĝé Enigmus