Grossular Garnet: Gemstone Information
Name Grossular garnet approved by the CNMMN of the IMA. Synonyms are grossularite, hessonite, essonite, cinnamon stone, rosolite, tsavorite, tsavolite, wiluite and mali. Colour varieties are hessonite (orange-brown), tsavorite (green) and mali (yellowish green to greenish yellow), leuco-garnet (colourless), imperial garnet (light pink), raspberry garnet (raspberryred), gooseberry garnet (gooseberry-green), tangerine garnet (bright orange), merelani (mint-green). ‘Grandite’ is the term that has been applied to grossular garnets with significant, although normally less than 50%, andradite molecule. Grossular is so named in allusion to the resemblance of pale green specimens to the gooseberry (Ribes grossularia).
Grossular is especially characteristic of both contact (skarns) and regionally metamorphosed impure calcareous rocks or rocks that have undergone calcium metasomatism. It also occurs in association with serpentinites and rodingites. Additionally as detrital grains in placers deposits. Notable localities for gem-quality grossular include Sri Lanka (hessonite), India, Tanzania (tsavorite, hessonite), Kenya (tsavorite), Mali, Namibia, Madagascar (hessonite), Pakistan, Russia (chrome-grossular), Italy, USA (California), Canada, Mexico and Brazil. Massive material similar in appearance to hydrogrossular occurs at: Yukon, Canada; California and other states in the USA; Myanmar; and South Africa.
Grossular Garnet forms a series with andradite and uvarovite, and also the hydrogarnets. Moreover, solid solution to varying extent occurs with the pyralspite series. Consequently the physical and chemical properties and
appearance can vary appreciably.
The RI of ugrandite garnets is strongly dependent on the calcium content, but does not vary significantly with changes in the Fe3+: Cr3+ ratio.
The calculated value of the end-member is 1.732; however, actual values will vary due to isomorphous replacement and measured literature values for 50% 100% grossular range from 1.796 to 1.737 respectively based on predominant andradite substitution.
Mali: 1.770 (greenish yellow); 1.768 (brownish yellow)
Mali pale greenish yellow: 1.739–1.750
Colourless Mali grossular: 1.742–1.751
Emerald-green grossular (Bekily, Madagascar): 1.741–1.746
Gogogogo, Madagascar: 1.742
Manson and Stockton study of 105 grossular garnets gave range from 1.731 to 1.754.
Refractive index values for African tsavorite range between 1.739 and 1.744 whilst that Swat material is 1.743.
Hessonite from Paskema and Okkampitiya, Sri Lanka, ranges from 1.731 to 1.735.
Mali: 3.66 (greenish yellow); 3.64 (brownish yellow)
Mali pale greenish yellow: 3.61–3.63
Emerald-green grossular (Bekily, Madagascar): 3.58–3.62
Gogogogo, Madagascar: 3.62
Manson and Stockton study of 105 grossular garnets gave range from 3.57 to 3.67.
African tsavorite values range from 3.57 to 3.65 and that of Swat material is 3.64.
Hessonite from Paskema and Okkampitiya, Sri Lanka, ranges from 3.598 to 3.622.
Hardness Tsavorite -7.25
Grossular garnets range from colourless (rare) through hues, shades and various tints of brown, yellow, orange and green. Mali, West Africa, for example, has provided gemmy rough in a range of colours including dark brown, orangy brown, light yellowish brown, yellowish green, light peridot-green, olive-green and intense bluish green. A direct correlation exists between increasing green tone and vanadium concentration; however, chromium, even in very low concentrations, can have significant effect on the green colouration. With increasing Fe3+, colour changes from yellow through orange to orange-red.
Originally discovered (1967) and described from the Tsavo National Game Park in Kenya from which it took its name, this green vanadium-bearing (goldmanite component) grossular garnet, with minor chromium, is more
prevalent in Tanzania where production from Tunduru, Ruangwa, Umba, Merelani Hills and Komolo has outstripped that from Kenya. Also found at Gogogogo, Madagascar. Colours range from bluish green to yellowish green, the former, sometimes called forest green, being the most highly regarded. Tsavorite is uncommon in sizes above 3 ct.
Vanadian grossular has also been reported from Swat, Pakistan. Meralani Mint Green This is the trade name for bright green garnet discovered in the same mines that produce Tanzanite, about 25 miles south-east of Arusha,
Tanzania. It is essentially a light coloured tsavorite.
Hessonite is the name given to the yellow orange to brownish orange colour variety. The colour is attributed to traces of iron. Major localities are Sri Lanka and India. Lighter oranges, yellows and peachy-coloured stones occur in deposits in Alberta and Quebec, Canada. Other localities include Mexico, Brazil, Tanzania and Madagascar.
(colourless) Leuco garnet is the name that is normally applied to the colourless, transparent variety of grossular that usually contains 2% iron. Main occurrences are Sri Lanka and Quebec, Canada.
This is the trade name for bright orange garnet, sometimes confusingly termed ‘mandarin’ grossular that has been found as water-worn alluvial pebbles from the Ruvuma River, Tanzania.Orange grossular from Tanga has been confusingly termed ‘malaya’.
In September 1994 a new deposit of garnet was discovered in the republic of Mali, in Western Africa. At first identified as chrysoberyl on the basis of colour, it was later found to be a combination of andradite and grossular (Gr80An20). The bright colours range from chartreuse to a yellow-green to an almost honey green. A rare chrome green colour has also been reported.
Apatite inclusions are common and calcite has been reported from a number of localities. Hessonites usually have a characteristic grainy and/or swirly internal appearance known as ‘treacle’: latest examinations show that this is the result of a mosaic structure caused by intergrown hessonite grains. Scapolite inclusions have been reported in massive hessonite from Maligawila, Sri Lanka, and also from Lelatema, Tanzania. Lamellar growth structures in Mexican hessonite produce iridescent colours similar to that reported for Mexican andradite. In tsavorite, fingerprint and geometric patterns arising from tiny droplets, negative crystallites and solid particles are reported. Additionally, graphite scales and asbestiform actinolite fibres have been observed.
Optical Effects: Chatoyancy, Asterism, Colour Change, ADR
Ultraviolet Fluorescence: some green garnets (grossular and tsavorite) may show a weak orange in LWUV and weak yellow in SWUV. A colour shift has been reported from Mali garnets: under incandescent light they have a more yellow tint, and in daylight they a
re more green.
Some colourless to pale tone grossular from various localties in East Africa will turn light yellow-green when irradiated (gamma rays) but will fade to their original colour within hours to days in daylight and within two months in the dark. These are characterized by a transmission window near 560 nm. Some material has been dyed green to imitate jadeite; this shows an absorption band from 630 to 670 nm and red under the Chelsea Colour Filter.