By Yaĝé Enigmus
The second series of major gem garnets is more diversely coloured and somewhat less common than the pyralspite garnets. This series is known as the “ugrandite” garnets, and it consists of uvarovite garnet, grossular garnet, and andradite garnet. The first two minerals of this series, uvarovite garnet and grossular garnet, represent the most monochromatic and most varicoloured ugrandite minerals respectively.
Uvarovite garnet is the calcium chromium nesosilicate member of the garnet group and the rarest species of gem garnet. Discovered in the Ural Mountains of Russia by Western scientists in 1832, this garnet mineral is named after Russian statesman and mineral collector Count Sergei Semenovitch Uvarov. Uvarovite is the only gem garnet known to always display green colouration, which is produced by the trivalent chromium (Cr3+) that is part of its pure chemical structure. Relative to other garnet species, uvarovite crystals are rather small and are almost never large enough to facet, but pieces of matrix coated in well formed crystals may be used in jewellery as “druzy” gemstones.
An extremely rare and exceptionally large uvarovite garnet gemstone, 0.44ct. Image: IGS/ Rob Lavinsky
Grossular garnet is the calcium aluminum nesosilicate member of the garnet group and is perhaps the most varicoloured garnet species. The name for this garnet mineral is derived from Grossularia, the genus name for many gooseberries, in reference to the gooseberry-like yellow-green colour of specimens once found in Siberia, Russia. The gooseberry garnets of Siberia are thought to be coloured by slight impurities of ferric iron (Fe3+) in combination with even slighter impurities of trivalent chromium (Cr3+); grossular garnet may simply appear yellow when only very slight impurities of ferric iron (Fe3+) are present, but this garnet species also occurs in a wide range of other hues.
Yellow-green “gooseberry” grossular garnet crystals on matrix from the Sakha Republic of Siberia, Russia. Image: Mindat/ Gerard van der Veldt
When grossular garnet is pure and/or contains no chromophores, it is completely colourless. In fact, grossular garnet is the only known garnet mineral to naturally occur in gem-grade colourless crystals. This particular variety of grossular garnet is known as “leuco garnet”, a name derived from the Greek word “leukos” (“λευκός”), meaning “bright” or “white”. Colourless garnet is considered to be one of the rarest of all garnet gemstones and gem-grade material has only been found in a small number of locations, with notable occurrences including the Merelani Hills of Tanzania and the Jeffrey Mine of Quebec, Canada.
Faceted leuco garnet gemstones from the Merelani Hills of Tanzania. Image: Gemstone Magnetism
Perhaps the most common variety of grossular garnet, hessonite garnet is known for its cinnamon-coloured appearance, often displaying a mixture of red, brown, and orange hues. The name for this garnet variety is derived from the Greek word for “inferior”, “hesson” (“ἣσσων”), referring to the slightly lower hardness of grossular garnet relative to pyralspite garnets. This garnet variety has been used as a gemstone in South Asia since ancient times where it was known as “gomed” and considered to be of equal or greater value than zircon; hessonite and zircon gemstones are similar enough in appearance that gems once thought to be zircon were revealed to be hessonite garnet upon closer analysis. Multiple different trace elements contribute to the colours associated with hessonite garnet: yellow and brown hues are caused by impurities of ferric iron (Fe3+), red hues are caused by impurities of ferrous iron (Fe2+), and orange hues are caused by the presence of divalent manganese (Mn2+) impurities.
An orange hessonite garnet cluster from the Jeffrey Mine, Quebec, Canada; Image: Mindat/ Weinrich Minerals
Tsavorite Garnet (and Chromian Grossular Garnet)
Grossular garnet may occur with rich green colouration; this variety of grossular garnet is known as “tsavorite garnet” and it is one of the most valuable garnet gemstones. Tsavorite garnet is named after Tsavo National Park in Kenya where geologist Campbell R. Bridges discovered a significant deposit in 1971 after first encountering the stone four years prior in northeast Tanzania. The colour of this grossular garnet variety, which varies from grass green to emerald green, is produced by impurities of trivalent chromium (Cr3+) and/or trivalent vanadium (V3+); the name for this grossular garnet variety is usually only applied to well saturated stones, while lightly-coloured stones are distinguished from tsavorite garnet with the name “mint garnet”. The geologic region which straddles the border of Kenya and Tanzania is the only significant source of tsavorite garnet gemstones, although a notable quantity of richly-coloured chromium-bearing grossular garnet was discovered at the site of the Jeffrey Mine in Quebec, Canada; despite nearly identical chemistry, green stones from the Jeffrey Mine are commonly referred to as “chromian grossular garnet” or “chrome grossular garnet” to distinguish them from tsavorite garnet, as the latter is a name usually reserved for the brightly-coloured stones mined in Africa.
In rare cases grossular garnet may exhibit a pink colouration. This particular variety of grossular garnet is known as “rosolite garnet” (not to be confused with rhodolite garnet), but may also be called “landerite garnet” or “xalostocite garnet”. The name for this grossular garnet variety is derived from the Spanish word “rosa” meaning “rose-coloured”. Well-formed rosolite crystals have only been found in two locations to date: the Jeffrey Mine in Quebec, Canada, and Sierra de Cruces in Coahuila, Mexico. Stones from the Jeffrey Mine exhibit light peach pinks due to the presence of both divalent and trivalent manganese, although most crystals found in this location are quite small. Stones from Sierra de Cruces may also exhibit peach pinks, but more often exhibit the deep raspberry pinks for which they are most famous. However, these deeper-hued gems are highly included; many rosolite crystals from Sierra de Cruces also show a black core due to the presence of quadrivalent titanium which takes part in a charge transfer with divalent manganese (Mn2+-Ti4+).
Raspberry pink rosolite garnet crystals on matrix from Sierra de Cruces, Mexico; Image: Mindat/ Allan Young
Grossular garnet may form with some silicate (SiO4) replaced by hydroxide (OH). This form of grossular garnet is known as “hydrogrossular garnet”, and it occurs in nearly as many colours as its non-hydrous counterpart. Hydrogrossular garnet most famously may contain impurities of trivalent chromium (Cr3+) or trivalent manganese (Mn3+), which cause the stone to display greens and pinks respectively. This variety of grossular garnet is most often cut en cabochon, as it virtually never exhibits true transparency, although high grade material can still produce attractive faceted gemstones with a soft, sleepy appearance. Green hydrogrossular garnet of a certain quality may sometimes be sold under the name “transvaal jade” due to high degrees of similarity between its appearance and that of jadeite and/or nephrite.
Rough green hydrogrossular garnet with black chromitite from South Africa; Image: Wikimedia Commons
In the next part of this series, learn about the final member of the ugrandite garnets, andradite garnet.
The Skyjems collection features richly-hued grossular garnets!
© Yaĝé Enigmus