Rhodolite  GARNET      Demantoid     Spessartine
(ALMANDINE, RHODOLITE, PYROPE, DEMANTOID, TSAVORITE, HESSONITE, GROSSULAR, SPESSARTINE)

    From the latin granatus, meaning "seed-like" or "having many seeds". The name is probably with allusion to the appearance of common, small red garnets which, due to their equant shape and color, reminded early observers of pomegranate seeds. The very earliest descriptions of garnet date to Roman times; Pliny the Elder described a mineral he referred to as carbuncle (a term still used today), a sparkling, glowing, red mineral, that was probably garnet. Of the more interesting lore associated with the curative or mystical qualities attributed to garnets, some southwestern U.S. indian tribes are said to have used red garnets as bullets, believing that their blood-red color rendered them particularly deadly and capable of inflicting fatal wounds. European apothecaries in the middle ages believed red garnet capable of relieving fever if applied with a poultice; yellow garnet was used similarly to treat jaundice.

    Like tourmaline, garnet comprises a complex group of minerals. All have the general formula R3M2(SiO4), where R=Ca, Mg, Fe2+, or Mn and M=Al, Fe3+, or Cr. One group of garnets, called Pyralspites, are named according to the dominant R cation present: Mg is Pyrope; Fe2+ is Almandine, Mn is Spessartine. The dominant M cations in these garnets is Al, with some Fe3+ usually present. A second group of calcic garnets (R=Ca), called Ugrandites, are named according to the dominant M cation present: Cr is Uvarovite; Al is Grossular; and Fe3+ is Andradite. Natural garnets are rarely pure (e.g. natural almandines usually contain variable amounts of Ca, Mg and Fe3+) and as a result these names apply strictly only to idealized ("end member") garnet compositions. Nevertheless, mineralogists apply these names according to which one most closely matches the composition of the garnet.

    Most gemologist do not have at their disposal a means for precisely determining the composition of a garnet, and therefore rely on color and refractive index for classification. Red stones are variously referred to as Almandine, Pyrope, or Rhodolite, the latter being a name that was originally used for lavender-red or violet-red garnet from North Carolina that is intermediate in composition between pyrope and almandine. The chromophore in these garnets is Fe. Pure pyrope is extremely rare and would be colorless (it is allochromatic); most red gem garnet called pyrope contains an appreciable almandine component. Spessartine is an orange or reddish-orange garnet (chromophore is Mn + Fe), although some varieties of mixed grossular-spessartine-pyrope garnets, notably Malaia garnet (see below), are also this color. A rich brownish-yellow variety of grossular with a color resembling topaz, is called Hessonite and a vivid green variety, containing chromium (and/or vanadium) and resembling fine emerald but with better brilliance, is called Tsavorite. Demantoid is a rare, more yellowish-green variety of andradite. Each of these garnet names has a corresponding narrow range of R.I., and it from this property that names are most frequently assigned.


Properties

  • Crystal System - Isometric (Cubic)
  • Habit - Usually euhedral.  Dodecahedra common.
  • Hardness - 6.5-7.5 (reds 7-7.5; others softer)
  • Cleavage - None
  • Fracture - Conchoidal
 
  • Specific Gravity

  • Refractive Index

Spessartine 4.15 (4.12-4.20)  1.81 (1.79-1.814)
Almandine 4.05 (3.93-4.30) 1.79 (1.76-1.82)
Andradite 3.84 (3.77-3.88) 1.89 (1.88-1.89)
   Demantoid  3.84 (3.81-3.87) 1.888 (1.85-1.89)
Rhodolite 3.84 (3.74-3.94) 1.76 (1.75-1.77)
"Malaia" 3.81 (3.78-3.85) 1.76 (1.742-1.78)
"Pyrope" 3.78 (3.62-3.87) 1.74 (1.71-1.77)
Grossular 3.61 (3.57-3.73) 1.74 (1.73-1.76)
   Hessonite 3.65 1.74-1.75
   Tsavorite 3.64 1.742-1.744
  • Color: All shades of red and pink; also yellow, green, black, brown, orange (and blue?).
  • U.V. Fluorescence: None, except in some green garnets (grossular and tsavorite) which may show a weak orange in long u.v. and weak yellow in short u.v..

Distinguishing Properties

Isotropic - most garnets will remain black in all orientations under a polariscope but some will not. These garnets possess an anomalous double refraction (A.D.R.) due to straining of the crystal lattice. Ugrandites are commonly A.D.R., as are some rhodolites and some almandines.

  • From ruby by lack of fluorescence, dichroism and double refraction
  • From red spinel by R.I.
  • From topaz by R.I.
  • From emerald by single refraction

Occurrence and Sources

    One of the more common rock-forming minerals in medium to high temperature, aluminous metamorphic rocks. Formed by contact or regional metamorphism of shales and limestones. Contact metamorphism of limestones is main source of gem quality, ugrandite series garnets. Those garnets formed by the metamorphism of shales, though common, are usually highly included and are not of gem grade. Most almandine and rhodolite is of this origin, however. Garnets rich in pyrope are found in kimberlite (the source rock for diamond) and some high temperature and pressure metavolcanic rocks. Gem spessartine is known only from pegmatites. Currently the most prolific production of gem garnet is from East Africa. Major discoveries in the 1970's and 1980's of rhodolite, tsavorite, and malaia garnet have resulted in the resurgent popularity of this gem mineral.

  • Almandine
    • Dark red to brownish or purplish red; the most common of all garnets.
    • Most on the market come from mica schist alluvial deposits in India (Jaipur area).
    • Other sources are Sri Lanka, Brazil, Idaho (some 4-rayed star stones), New York, N. Carolina and Alaska.
  • "Pyrope"
    • Dark red (blackish-red) Mg-rich garnet; most contain a component of almandine.
    • "Bohemian" garnets, popular in late 1800's are "pyrope". Are found in conglomerates, volcanic breccia, tuffs and alluvial deposits.
    • Color-change pyrope from Norway and Tanzania show a change from violet in sunlight to red in tungsten light.
    • Main localities for pyrope are Czechoslovakia (Bohemia; since 1500 AD); South Africa; Otteroy, Norway; Tanzania, and Arizona. Also localities in Australia (New South Wales, Anakie), Myanmar, Argentina, and Brazil.
  • Rhodolite
    • Original material from Macon Co., N. Carolina has a distinctive purplish red ("rhododendron red") color with almandine to pyrope ratio of 2:1.
    • Best are a lovely violet or purplish-pink, reminiscent of fine pink sapphire or rubellite.
    • Some Tanzanian rhodolite shows a color change from bluish green in daylight to the more typical purple red in incandescent light.
    • East Africa now main source (Tanzania, Kenya); also Madagascar (color change blue to burgundy is of this type?), Sri Lanka (noted for higher priced, pure violet red stones), Zimbabwe, and India
  • Spessartine
    • Lovely orange, yellow, or flame red color, usually small. Color can resemble some hessonite garnets, with which they are sometimes confused.
    • Rare, main sources are (were) at Amelia, Virginia, San Diego Co., California (Little Three Mine), and Brazil.
    • "Mandarin" garnet is spessartine from Namibia that has gained popularity in the past decade. The exceptional orange color commands considerably higher prices than other spessartine.
    • Others include Idaho, Norway, Pakistan, Madagascar, Myanmar, Sri Lanka and New South Wales, Australia.
  • "Malaia" (also "Malaya") Garnet
    • Name of recent vintage used for pinkish orange, reddish orange, yellowish orange, or golden pyrope-spessartine-grossular-mixed garnet from the Umba River valley bordering Tanzania and Kenya.
    • Best are said to have a pinkish or pure-orange color, but a browner orange resembling hessonite or spessartine is more common.
  • Grossular
    • Shades of yellow, pink, green and pale brown.
    • Important varieties include Hessonite (cinnamon-stone), a brownish yellow, orange yellow, or brownish orange color, and Tsavorite (or Tsavolite), a bright, vivid lime green color.
    • Tsavorite typically contains rounded inclusions of apatite and calcite.
    • Tsavorite found only near the Kenya/Tanzania border, near Tsavo National Park, in a graphite schist. A similar lime-green grossular is found in the Umba Valley in Tanzania. Inclusions in stones from this locality are graphite and limonite-stained cracks. Gem green grossular garnet is also known from Pakistan.
    • Most Hessonite from Sri Lanka; also from Brazil, Vermont, Italy, Mexico, Quebec.
  • Andradite
    • Only important variety is Demantoid, a very rare, brilliant yellow-green andradite.
    • Has very high dispersion (0.057; higher than diamond) and high R.I.( 1.89); accounts for brilliance, also name (demant is Dutch for diamond).
    • Hardness only 6.5, as compared to 7-7.5 for red garnets.
    • Contains characteristic "horse tail" inclusions of a fibrous amphibole (byssolite) that are diagnostic for Russian demantoid.
    • Finest demantoid from the Ural Mts., Russia, where it occurs in serpentinite. No present production.
    • Gems of 4 carats or larger are extremely rare.
    • Other sources are Italy (very small stones), Switzerland (Zermatt; little to no gem material), Zaire and Mexico (Piedra Parada; little to no gem material).

Pricing and Valuation

  • Top quality stones were in the following ranges (wholesale) in 1990:
  • Almandine - $10-$50/ct for 5 carat sizes.
  • Pyrope - $10-$30/ct for 5 carat sizes.
  • Rhodolite - $50-$100/ct for 5 carat sizes.
  • Spessartine - $50-300/ct for 5 carat sizes.
  • Tsavorite - $600-1200/ct for 2 carat sizes.
  • Demantoid - $1000-$4000/ct for 1-2 carat sizes; $3000-$6000/ct for 2-3 carat sizes.
  • Malaia - $100-200/ct in 5 carat sizes.

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1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin