The six common varieties form two composition-related groups (solely for classification purposes):
in which there's a complete solid solution series between the following species:
- aluminum garnets, pyralspites (pyrope, almandine, spessartine) - the vast majority of gemstone garnets
- calcium garnets, ugrandites (uvarovite, grossular, andradite) - widest color range
and only partial series between the two groups. Garnets come in every color except blue - though some of the color-change ones do pass through a blue phase under certain lighting conditions.
- pyrope : almandine : spessartine
- andradite : grossular : uvarovite (may be only partial series)
The less common varieties are:
See MinDat's page for a complete listing of the Garnet Group.
Generically, garnets have the formula: A3B2(SiO4)3 where A can be Ca, Fe (ferrous), Mg, Mn (manganous) and B can be Al, Fe (ferric), Mn (manganic), Cr, Ti, V, Zr with iron and aluminum occasionally replacing some of the silicon. The number of combinations is reflected in their diverse color range. To include the synthetic garnets, the formula can be rewritten as A3B2(CO4)3, or even A3B5O12 since in synthetic garnets, B and C are usually the same element. Here, A can also include bismuth, gadolinium, and yttrium and B can include gallium and vanadium.
Some pyropes and spessartines are color-change garnets (thanks to Joel Arem's Color Encyclopedia of Gemstones and others):
|Locality||Incandescent / Daylight, Fluorescent (transmitted / reflected)|
|North Pare Mountains, Tanzania||purplish red / blue|
|Bekily, Madagascar (1999)||plum red / bluish-green|
|Norwegian||wine red / violet|
|Wellawaya, Sri Lanka (1996)||purplish red / dark bluish green|
|Tunduru, Tanzania||intense red / greenish gold|
|Tanzania||tan / light green|
|Tanzania||purplish red / brownish green|
|Umba Valley, East Africa||magenta / greenish-blue|
|Voi, Kenya||pinkish orange / medium brownish yellow|
|East Africa||reddish orange to red / greenish yellow-brown / purplish red|
|East Africa||light red to purplish red / light bluish green / purple|
|Unknown||violet-red / blue-green|
|Russia||olive-green / orange [photo courtesy of Thai Gems]|
There's also some mixed garnets (two or more species on one matrix) and some combination garnets. Most garnets are not purely one species but the combination ones here are known to be "midway". Malaya (Malaia) garnets are a mix of pyrope, spessartine, and grossular (significant vanadium impurities create the color-change variety), rhodolites (and Mozambiques - red w/ orange flashes; the mine is now closed) are a combination of pyrope and almandine, and Mali garnets (1994) are a rare mixture of andradite and grossular (grandite). Grape (violet to purplish-red) garnets are almandine-spessartines from the Orissa district of northwestern India. Some garnets can be combinations of more than three species - in 1994, in Slovakia, there's an unusual vanadium-chromium garnet that's part goldmanite, uvarovite, grossular, and yamatoite.
Synthetic and homocreate garnets
I also have a few lab-produced (faceted) garnets - these garnets can be either homocreate (same composition as natural garnets) or synthetic, that is, not having a natural counterpart. Homocreated created garnets are relatively difficult to fabricate and since, in general, natural garnets are relartively cheap, there's not much incentive to manufacture them. "Synthetic garnets" (especially with the same garnet name as the natural ones) for jewelry are almost always colored synthetic spinels. Most of the true synthesized garnets are doped YAGs - yttrium aluminum garnets. I have a cesium-doped (yellow) and a chromium-doped (green) YAG. Both are highly fluorescent - the green one has a green-red color change.
Synthetic garnets have other uses than gemstones - yttrium-iron garnets (YIG) are magnetic and find many uses as sensors, actuators, and microwave substrates.
Finally, there's a few unidentified garnets here.
From the abstract of A natural scandian garnet. in American Mineralogist, Volume 90, 1688 - 1692, 2005.
Garnet from an aposkarn achtarandite-bearing rodingite-like rock in Sakha-Yakutia, Russia, has a Sc content close to 6 wt% Sc2O3 . . . Investigation of the composition of many of the scandian garnets reveals the existence of a solid-solution between kimzeyite-schorlomite Ca3(Zr,Ti)2(Al,Fe)2SiO12 and the scandium analog of andradite Ca3Sc2Si3O12. This is the first report of a natural scandian garnet.
From the abstract at Neues Jahrbuch fur Mineralogie-Monatshefte 1999, Vol , Iss 3, pp 123 - 134:
"Schaferite, a new member of the garnet group, has been discovered in a fragment of a silicate-rich xenolith from the Bellberg volcano near Mayen, Eifel, Germany. . . . Schaferite has an orange-red colour and a vitreous lustre, the streak is yellow. . . . the simplified formula as derived from chemical analyses and crystal-structure investigation is NaCa2Mg2(VO4)3."
The "vanadate" allies it with the goldmanite-yamatoite subgroup.