Pyrope Garnet: Gemstone Information
The Pyrope Garnet Name approved by the CNMMN of the IMA. Synonyms include names incorporating ruby prefixed by locality, for example Arizona ruby, Bohemian ruby, Cape ruby and Elie ruby. Named in 1803 by Werner from the Greek word for fire and to appear in allusion to its characteristic red colour. The composition of the rose-red to violet-coloured variety, known as rhodolite, approximately equates two parts pyrope Garnet and one part almandine. The original material from Macon Co., North Carolina, USA, has a distinctive purplish red (‘rhododendron red’) colour. The dark brownish red pyrope–spessartine–grossular combination is referred to as malaya (or malaia).
Unlike other garnets the most common origin of pyrope is igneous rather than metamorphic. It occurs in peridotites, kimberlites, eclogites, serpentinites and other ultramafic igneous rocks and sediments derived from
these. Metamorphic pyrope arises from metamorphism of the aforementioned rocks or from magnesium-rich rocks subjected to high-grade metamorphism. Notable localities include Bohemia, Czech Republic and the Dora
Maira Massif, Western Alps, Italy. In the USA, the most significant deposits are in Arizona near San Carlos (in the San Carlos Indian Reservation), Gila and Graham counties and near Fort Defiance (Buell Park and Garnet Ridge), Apache Co. Notable deposits exist in the Four Corners area, where the four states of Colorado, Arizona, Utah and New Mexico meet. In Africa, pyrope is found in many of the South African diamond mines, along the Umba River and in the Pare Mountains, Tanzania, at Mandera, Hargeisa, Somaliland; the Lundaze area, Zambia and Madagascar. In Australia at localities in New South Wales and Queensland, and in Argentina at Quines. Small amounts of pyrope also occur in Sri Lanka and Minas Gerais, Brazil. In Canada, pyrope is found in Joli Township, Quebec, and in Russia at Urals, and Yakutia. Other European localities include Vetarella, Vico, Lazio, Italy; Gorund, Switzerland; Arguenos, France; Saxe, Germany; and Elie Ness, Fife, Scotland (‘Elie Ruby’).
The classic locality for rhodolite is Cowee Creek, Macon Co., North Carolina, USA. Rhodolite is also reported from Tanzania, India (Orissa), Sri Lanka, Malawi, Brazil and Madagascar. In Mozambique, red-violet rhodolite is found from Cuamba (previously Novo Freixo). Rhodolite has been reported from the Lokirima area, Kenya.
Malaya has been recorded from various localities straddling the Kenya–Tanzania border, Mozambique, Nigeria and also Madagascar.
There is complete solid solution amongst the garnets of the pyralspite series and pyrope commonly contains significant amounts of the almandine molecule. It can often contain small amounts of grossular. Chrome pyropes tend to have a greater percentage of pyrope molecule but all have 4% of the uvarovite (or knorringite) molecules. Consequently the physical and chemical properties and appearance can vary appreciably. Although less favourable in comparison with grossular, hydroxyl (OH) component has been recorded in pyrope. The purplish pink pyrope from the Dora Maira Massif, Italy, uniquely shows four sharp OH vibration bands in the infrared spectrum between 3660 and 3600 cm-1. Pyrope from the Monastery Mine, South Africa (containing 56 ppm H2O),
exhibits broad OH absorption between 3500 and 3600 cm-1. The quantities are negligible and have no discernable effect on the properties.
The RI of pyralspite garnets is strongly dependent on the magnesium content and is less influenced by the iron–manganese ratio. The RI of synthesized pyrope is 1.714 and the theoretical value reported as 1.705. The measured values of most natural pyrope vary due to isomorphous replacement, and literature values range from 1.73. The upper limit is more problematic due to variable cross-series isormorphous replacement and the value of 1.76 has been quoted for a pyrope of composition Py51Al37Sp5Gr7. The purplish pink pyrope from the Dora Maira Massif,
Italy, is unusually pure, up to Py97Al2Gr1, giving an RI range from 1.717 on the palest specimens up to 1.730 with increasing almandine molecule. The largest supposed Bohemian pyrope is in a Czech museum and weighs 13.21 ct; it has an almandine spectrum and RI of 1.79, which suggests almandine rather than pyrope. The RI range for rhodolite has evolved such that 1.740–1.770 is acceptable to some authorities. For example, the brownish red pyrope-almandine (Py52Al39Gr7An2) with a purple component from Kalalani, Tanzania, with RIs ranging from 1.763 to 1.77 has been termed ‘rhodolite’. Malaya garnets from Bekily, Madagascar, have an RI range for Py24-68
from 1.782 to 1.740. Malaya from Mozambique, which is an apricot/ orangy red coloured material, has an RI range from 1.751 to 1.755. Colour change pyrope-spessartine is obtained from Sri Lanka with RI 1.77.
Grape garnet describes an intense purple-red variety without delimiting RI boundaries although to date the RI range indicated by Hanneman is 1.765–1.805. Other authors cite RIs between 1.758 and 1.780 for pyrope-almandine from the Orissa state of North-west India, but are equivocal about linking the specific locality, colour and percentage of pyrope molecule, which ranged from 80 to 56%.
Pyrope has the lowest density of all the garnets. The calculated SG of the pure end-member is 3.563. The measured values of natural pyrope vary due to isomorphous replacement that can also be cross-series and
normally range from 3.65 to 3.84: the average being 3.74. The unusually pure purplish pink pyrope from the Dora Maira Massif, Italy, give values from 3.58 to 3.67 with increasing almandine molecule. Malaya garnets from Bekily, Madagascar, have an SG range for Py24-68 from 4.04 to 3.78. Malaya from Mozambique has an SG range from 3.844 to 3.847. Colour change pyrope-spessartine is obtained from Sri Lanka with SG 3.93. Specific gravity quoted for grape garnet from Orissa, India, ranged from 3.82 to 3.94 and expanded to 3.77 for pyrope-almandine with unrecorded colour.
Hardness on Mohs’ scale ranges from 7 to 7.5.
Pure pyrope is colourless; however, it is typically pinkish red, ranging from orange to brownish red, through an almost crimson colour, to a purplish red with increasing amounts of the almandine molecule. Small amounts of manganese can eliminate the purple component resulting in an intense red. Normal chromium pyropes are blood red with a purple tint, and with increasing chromium content the hue is lilac or purple passing ultimately to a greenish hue (knorringite). Pyrope with strong tonal qualities is blackish red. Fe2+ in pyrope garnet produces the near-red colour but often with an orange tint, whilst Cr3+ is responsible for producing a richer red colour similar to that seen in ruby or spinel, but the effect is muted by the ever present iron.