The Colourful World of Gem Garnets

The Colourful World of Gem Garnets

When we think of garnets, the color that comes to our minds is red. As a gem material, it has been known since antiquity and as a mineral, it was defined in a time when names were mainly given based on visual properties. Garnet comes from the Latin granatum, "pomegranate", that in turn is derived from granatus, meaning something along the lines of "with many grains" in a clear allusion to the similarity of the stone’s color with the color of the juicy grains of the fruit. Curiously, ruby in Thai is called "thabthim" precisely after this fruit also. The old name for red gem gar- nets was yet another one. Pliny, the Elder (23-79 CE), the famous Roman naturalist from Pompei, in his 37 volume work "Natural History" refers to them as carbunculus alabandicus that were known to occur namely in India and Portugal at the time. The term carbunculus, translated as anthrax in Greek, alluded to their resemblance to burning coal in transmitted light while alabandicus was a reference to the old city of Alabanda, Turkey, where these gems were known to be traded in Antiquity. Curiously this locality-related term is behind one of the names of a garnet mineral species that will be mentioned further ahead (almandine).

Although red is the immediate hue that we relate to garnet, the truth is that garnets do occur in many colors. Garnet is a collective name for a significant number of min- eral species and mineralogists gather them in what is called a supergroup where there are two gemologically relevant sub-groups: pyralspite (an isomorphic series of aluminium-rich garnets: pyrope, almandine and spessartine) and ugrandite (an isomorphic series of calcium-rich garnets: uvarovite, grossular and andradite). In mineralogy, an isomorphic series is comprised two or more mineral species, called the end-members, with similar crystal structures and slightly different chemical compositions due to the replacement of chemical elements without affecting the crystal structure, thus generating similar crystal forms, hence the name isomorphic from the Greek iso, "same", and morphos, "shape" or "form". The intermediate members of an isomorphic series are called "solid solutions" and virtually all gem garnets are solid solutions on the pyralspite and ugrandite series, meaning that they have mixed compositions and, therefore, varying gemological properties within certain limits. A few decades ago, to properly identify the garnet species or variety, standard gemological testing, namely specific gravity and refractive index measurements, was considered a reliable method. Today, due to the structural and compositional complexity of gem garnets, their identification is further supported by advanced analytical techniques, namely Energy Dispersive X-Ray Fluorescence (EDXRF), Raman spectroscopy and the even more sensitive LA-ICP-MS (Laser ablation-Inductively Coupled Plasma-Mass Spectrometry).

In the photo: Ecce Homo carving in hessonite garnet, probable singalese, late 17th century. © Carlos Pombo Monteiro / Fundação Eugénio de Almeida, Arquidiocese de Évora

The pyralspite isomorphic series includes vivid red to pink to orange gem varieties, with different iron-rich almandine, magnesium-rich pyrope and manganese-rich spessartine compositions. In the trade some of those garnets may be known as rhodolite (rose colored pyrope  with almandine component), mandarin garnet (vivid orange spessartine) and malaia garnet (pinkish orange spessartine with pyrope component). Interesting to know that this last one may often occurs in color change varieties, meaning that the color will look different under different light sources (daylight equivalent and incandescent light equivalent). Also interesting is the fact its trade name was originally dubbed in the Umba region (Kenya and Tanzania) from the Swahili malaia, meaning "prostitute" or "out-cast", since they were traditionally rejected for its unusual color. Fortunately, not the case anymore. Almandine garnets are usually brownish red and pyropes tend to more deep red colors. Historically, the chromium-bearing rich-red pyropes from central "Céske Středohoří in Bohe- mia, Czech Republic, with recorded extraction activity before the 14th century, were known in the trade as "Bohemian rubies". Typically in small sizes up to 6 mm, they were highly popular in the 18th and 19th centuries and are behind the establishment of the famous Gemstone Training School in the city of Turnov in 1884. Still today, local jewellers manufacture garnet jewelry in traditional Czech designs. Almandine-pyrope garnets are also known to occur occasionally with asterism, displaying a star effect usually in reflected light. Due to the cubic symmetry of the garnet crystals, four-rayed stars and six-rayed (even twelve-rayed) stars may be observed, sometimes both in the same specimen polished as cabochon or round beads. Photo bellow © Lang Antiques)

The ugrandite isomorphic series includes the chromium-rich vivid green uvarovite with limited gemological use (usually in drusy specimens), the aluminium-rich grossular and the iron-rich andradite, both with many gem varieties of different colors. 
Grossular garnets have been used in jewelry for ages. Its orange variety that came from Kamburupityia in Sri Lanka is known in the trade as hessonite and historically referred to as "cinnamon stone" or "jacinth", being already seen in Medieval jewelry. A nice example is illustrating the entry of this article as a late-17th century fine carving of Ecce Homo. Hessonite is gemologically very interesting as it often displays profuse crystal inclusions of apatite and a typical roiled appearance that is a delight to observe under magnification. Its most famous gem of this species is probably the chromium-bearing vivid green variety known in the trade as tsavorite. It was discovered and named in the late 1960s by the late Campbell Bridges (1937-2009) in northern Tanzania and was later located in Kenya in significant quantity. Light green grossular from Merelani, Tanzania, has been collected as a by-product of the tanzanite and is known in the trade as Merelani mint garnet. More colors may occur in grossular garnets, including pink (often known as rosolite), greenish yellow, yellow, colorless (leuco garnet) and some with color-change phenomena. An interesting variation with a rather complex composition, is the so-called hydrogrossular, or hydro garnet, characterized by a significant hydroxyl (OH-) component, being typically massive in texture and translucent to opaque in green to pink colors. The green varieties from South Africa were known in the trade by the misnomer "Transvaal jade".
In the photo, a 20.42 ct tsavorite garnet superbly cut by Mikola Kukharuk for Nomads, 2014 © AGTA Spectrum Awards
Andradite, named after Portuguese-Brazilian mineralogist and politician José Bonifácio de Andrada e Silva (1763-1838), also has many gem varieties, again with different colors. The most famous one is demantoid, a pale to vivid green chromium-bearing garnet with sub-adamantine luster, a characteristic that is behind its name, from the Flemish demant, "diamond", in a clear allusion to its diamond-like luster. It also has very high dispersion, meaning that white light is split into its spectral colors as it trav- els in and out of the stone offering the cut gem a colorful flashy visual appearance known as "fire". Demantoid was discovered in the mid-1800s in the Ural Mountains, Russia, becoming very popular among the Romanov imperial family and in the Rus- sian court. Gemologists and collectors value specimens with the diagnostic fibrous chrysotile inclusions that resemble a horsetail. These inclusions are wonderful to observe under magnification and may be seen in Russian material but also in stones from Eritrea, Iran and Italy. With less popularity in the jewelry industry there are black andradites, known in the trade as melanite, and yellow to brown varieties, known in the trade as topazolite.
In the photo, a late-19th century goose brooch featuring diamonds and demantoid garnets © Ruby Lane, Inc.
It is a challenge to condense an article on such a vast, varied, and complex group as garnets while managing to cover all the relevant aspects of this interesting gem family with known occurrences all across the globe with reported historical pres- ence in jewelry. Many trade names and historically interesting misnomers have been used throughout the ages to describe garnet group gem varieties and it is up to trade organizations like CIBJO - The World Jewelry Confederation - to establish accepted trade names for these gemstones and for the gemological laboratories to follow that nomenclature rules to properly address them to the trade and to the consumer.
Edited from Galopim de Carvalho, Rui (2019), "The Colorful Garnet World", Gamma - The ICA | GemLab Journal, Vol. 1, no. 1, pp. 28-32
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