Almandine Garnet: Gemstone Information
Almandine Garnet was named after the now ruined ancient city called Alabanda, 10 km west of present-day Cine, Aydin, south-west Turkey. From the writings of Theophrastus (315BC) and almost 400 years later of Pliny, this opulent city was known to be a cutting centre for quartz gems and garnet.
Almandine is the commonest species in the garnet group, typically occurring in schists and gneisses, resulting from regional metamorphism of largely clay rich sediments and rocks. It also occurs in granites and in granite pegmatites and some volcanic rocks, as well as in sedimentary deposits as detrital grains. Whilst occurrences are worldwide (total world industrial garnet production in 1999 was estimated to be 335 000t), the gem material is less abundant. India, for example, produced 677 kg of gem garnets in 2001–2002. Notable localities for gem-quality almandine include: South Africa, Zimbabwe, Tanzania, Madagascar, Kenya, Sri Lanka, Myanmar, Brazil, Argentina, Australia, Bohemia, Russia, Pakistan, India, Canada, Mexico and the US. In 1998, a new garnet deposit was found in Brazil; it is amethystine garnet and has been identified as almandine. It is located
in Fazenda Balisto in Tocantins.
Almandine generally contains significant amounts of both the pyrope and spessartine molecules forming a series with these, and may also have noteworthy amounts of the grossular molecule. Consequently the physical and chemical properties and appearance can vary appreciably.
The theoretical value of the end-member has been variously reported as 1.827 and 1.830. Most authorities quote 1.830 as the maximum value. Lower values are more problematic given ionic substitution between the other pyralspite series members, limited ugrandite components and the mineralogical dominant molecule compositional criteria. Bearing this in mind, assigning the name almandine to Fe/Al garnets with RI values lower than 1.773 should be avoided unless supported by chemical analysis. A rule of thumb is that almandine garnets have RI values greater than 1.78. Some authors, however, make reference to a lower limit for almandine as 1.75. This approximates to the lower limit of the ‘intermediate category’. The RI values for almandine garnets are not
exclusive with some grossular-andradite garnets approaching 1.78.
The limitations of practical gemmological gem testing and the unclear nature of the lower limit for almandine has prompted the acceptance of an intermediate category. This has been variously called ‘pyralmandine’ (proposed by Fermor), ‘rhodomacon’ (Campbell), ‘umbalite’ or ‘pyrandine’ (Anderson) and ‘pyrope-almandine’ (Stockton and Manson). None of these names has generated universal acceptance.
Pure end-member (calculated) 4.313. As with RI, the values for density vary with the amount of ionic substitution. Extracting values from the literature, where chemical information is provided, indicates that for pyralspite garnets containing 50% almandine molecule the lower limit of density approximates to 3.95. It should be borne in mind that inclusions can affect measured density and, if in abundance, give rise to altered values.
Generally held to be 7.5 on Mohs’ scale but has been reported to range from 6.81 to 7.48.
Colour is a significant consideration. The colour of almandine has been described as various shades of brownish red, orange-red, red and violetred. Colour is also related to size and large stones are darker. In order to lighten darker stones they have been cut as hollowed out cabochons called carbuncles: a practice introduced in Roman times. Colour Varieties: Almandine (allegedly) of a rich red grape colour has been reported from Tanzania, Madagascar and Brazil Colour is a significant consideration. The colour of almandine has been described as various shades of brownish red, orange-red, red and violetred. Colour is also related to size and large stones are darker. In order to lighten darker stones they have been cut as hollowed out cabochons called carbuncles: a practice introduced in Roman times.
Colour Varieties: Almandine (allegedly) of a rich red grape colour has been reported from Tanzania, Madagascar and Brazil.
Almandine is usually included with a variety of minerals. Commonest amongst these are needle-like crystals of rutile aligned parallel to the dodecahedral face. Planar intersections are at angles of 70° and 110°. This gives rise to two sets of long needles accompanied by a set of apparently shorter needles as the latter are inclined to the plane of the other two. Other common inclusions are euhedral crystals of zircon with accompanying tension halos and apatite. Less common inclusions include quartz, monazite, ilmenite, spinel, biotite, titanite (sphene), phlogopite, muscovite, plagioclase, chalcopyrite and sphalerite.
Optical Effects: Chatoyancy, Asterism, Colour Change, ADR
Chatoyancy and asterism: When in abundance, rutile inclusions can give rise to asterism. Four-ray stars occur when the needles are parallel to the edges of the dodecahedral faces and six-ray stars when parallel the edges
of the octahedral faces. Complete spheres show multiple stars. India, Emerald Creek area of Idaho, USA, and Ambatondrazaka, near Lake Alaotra, Madagascar, are reported as sources of star almandine garnet. The star garnets from Ambatondrazaka, Madagascar, are intermediate members of the almandine-spessartine series with an appreciable pyrope component. Almandine from the latter is also reported to exhibit reddish brown cat’s eyes arising from orientated sillimanite lamellae. Star garnet has also been reported from Sri Lanka but the species has not been defined. There is no dichroism, although there may be anomalous double refraction.