Lets Talk Gemstones

By Edna B. Anthony, Gemologist
(Contact the author for permission to reproduce this article in any form.)
P.O.# 62653; COLORADO SPRINGS, CO. 80962




The garnets are a complex group of nesosilicates of the silicate class of minerals. In nesosilicates, only ionic bonds formed with interstitial cations (positive charged atoms) connect the isolated SiO4 tetrahedra. The size and the charge of these cations generally determine the structures of these minerals. An equidimensional crystal habit and a lack of distinct cleavage planes are the result of the independence of the SiO4 tetrahedra. Dense atomic packing causes the characteristic high specific gravity and hardness of the structures. In the twentieth edition of the Manual of Mineralogy by Cornelis Klein and Cornelius S. Hurlbut, Jr. after J. D. Dana, we are told that garnets “crystallize in the hex octahedral class of the isometric crystal system.” The most common crystal habits for this class are the cube and the octahedron. The arrangements of the atoms in their structures are such that these habits are rare in garnet. (It is interesting to note that only pyrope occasionally exhibits cubes with curved faces.)

Garnets usually occur in the dodecahedral and trapeszohedral forms or combinations of these forms. The dodecahedral form is so typical that the dodecahedron was once known as a garnetohedron. Richard M. Pearl states in his Garnet, Gem and Mineral that “Twinning of garnet crystals shows only in the effect of double refraction.” This anomalous double refraction may indicate internal strain that may cause complex or sector twinning. (“Sector twins consist of 12, 24 and 48 pyramids meeting at the center of the crystal.” -quoted from Pearl) However, some mineralogists think that such double refraction in garnet may be evidence of crystallization in the tetragonal crystal system. In the Color Encyclopedia of Gemstones, Dr. Joel Arem presents an excellent diagram of the relationships between the garnets. Under the formulas for the garnet species, one is instructed to note “Henritermierite: Ca3(Mn,Al)(SiO4)2(OH)4. Tetragonal, very garnetlike, often twinned.”

A3B2(SiO4)3 can represent the structural formula of garnet. The 8 coordinated cationic sites represented by A are occupied by rather large divalent cations. B represents 6 coordinated cationic sites occupied by smaller trivalent cations. In garnets, the A cationic sites can be occupied by the large divalent atoms of calcium, magnesium, iron or manganese. The 6 cationic sites represented by B are occupied by smaller trivalent cations of aluminum, chrome, or iron. The chemical compositions of the garnets allow them to be grouped in two series known as isomorphous series. One of the series is composed of garnets, where calcium atoms occupy the A sites. This series includes uvarovite, grossular, and andradite and is referred to as the ugrandites. Arem notes that the large atoms of calcium in the structure of the ugrandites cause them to exhibit birefringence. X-ray data reveals that the ugrandites can crystallize in the orthorhombic and, perhaps, in the monoclinic crystal systems. The occupation of certain crystallographic sites by specific cations may cause such crystallization. Twinning occurs frequently in andradite and grossular garnets, and color zoning is the norm. Hydrogrossular is formed when tetrahedral (OH4) groups (hydroxyl) replace some of the SiO4 tetrahedra in the grossular composition. The water content of some hydrous garnets may be as much as 8.5%. Melanite, the black variety of andradite, develops when sodium replaces calcium and Ti4+ enters the B cationic sites.
In the other garnet series, no calcium is present (magnesium, iron, or manganese atoms occupy the A sites) and the B sites are occupied by aluminum cations. This series is known as the pyralspites and includes pyrope, almandine, and spessartine. Pure end members of either series are seldom found. Extensive substitution occurs in each of the series, but there is little solid solution between the two series. Richard M. Pearl mentions such a combination called spandite. It is a link, which involves titanium, between spessartite and andradite. He also makes the statement that, “As long as any chemical element can fit into the atomic structure because its size is right, the composition of garnet is variable.” Phosphorus, vanadium, yttrium, and zirconium are other elements that sometimes replace atoms in garnet’s structure.

Garnet is a common mineral distributed worldwide. It occurs as crystals, in massive and granular forms, and as tumbled pebbles. It can form under a wide variety of geological conditions, but high temperatures are essential for its development. It is of major importance as a rock-making mineral in igneous, metamorphic, and sedimentary rocks. It alters frequently to chlorite, serpentine, and talc. Chemical stability and resistance to weathering permit excellent crystals to be found in alluvial deposits. It is known that inhabitants of the American southwest still recover crystals from the desert sands and ant hills there. Garnet has imperfect cleavage, but it can exhibit an unusual angular fracture.   This ability allows it to retain sharp cutting edges. Industry takes advantage of this property and its hardness to produce abrasive papers and cloths that are two to six times more efficient than those of quartz. The large crystals of almandine recovered at Gore Mountain in New York provide a major source for this industrial use. The physical properties of pyrope (its elasticity and heat conductivity) make it ideal for bearings used in the manufacture of very accurate watches, clocks, and other fine instruments.

The use of garnet as a gemstone is historic. Before the technique of faceting was developed, material from the underside of well-formed domed crystals was often removed to facilitate the transmission of light through the stone. It is known that garnet was used before 3400 B.C. in Predynastic Egypt and in Sumeria as early as 2350 B.C. Artisans of the Bronze Age (2000 to 1000 B.C.) in Sweden incorporated garnet in their works. Caravan traders with sources in Africa brought to Carthage garnets that were highly prized in Rome. Pliny, quoted from an early Hebrew writing, said, “for the traveler the well formed image of a lion, if engraved on a garnet will protect and preserve honors and health, cures the traveler of all diseases, brings him honor and guards him from all perils incurred in traveling.”

The Persians frequently carved images of their great men on garnets. The inhabitants of the Middle East regions and Egypt obtained garnets from India as early as 1000 B.C. through trade with Arabia. A garnet was one of the twelve gems mounted in the breastplate of Aaron, sacred to Jews, which symbolized the twelve tribes of Israel. Some of the peoples of Asia used garnets as “magic” bullets. They believed such missiles were more accurate and lethal. Relics of the Aztecs in central Mexico show they used garnets frequently.

In the American southwest, the Pueblo Indians began to use them as gems in their later works. Garnets collected by the Comanche Indians at Jaco Lake in Chihuahua, Mexico have been found at the Pueblo of Picuris in New Mexico. We know this pink glossularite recovered from white marble deposits at Xalostoc, Lake Jaco, and Morelos, Mexico as xalostocite, rosolite, and landerite.

In Europe, pyrope from deposits in Bohemia supplies some of the finest gems to jewelers. The Victorian era is renowned for the use of these gems. Melanite was used extensively in “mourning” jewelry during this period.

Attempts to synthesize garnet for industrial purposes began during the 1960’s. These materials possess the structure of natural garnet but differ in chemical composition, and they have no counterpart in nature. YAG (yttrium-aluminum-garnet) is produced in a range of colors and colorless. Its dispersion exceeds that of diamond. Faceted YAG is frequently used as a diamond substitute. Twenty-eight of its trade names are listed on page 234 of the second edition of the Color Encyclopedia of Gemstones by Dr. Joel Arem. GGG (gadolinium-gallium-garnet) also serves as a diamond imitation. YIG (yttrium-iron-garnet) is opaque and black with a metallic luster and is sometimes used by the trade to imitate hematite.

Most sources indicate the name garnet is derived from the Latin word granatus meaning “like a grain.” Before the science of mineralogy developed, most red gems (including garnet) were known as carbuncles, also from the Latin meaning “a live or burning coal.” Natural garnet is known by numerous appellations. Some are more familiar than others. The glossary compiled and published by Richard M. Pearl is enlightening. It seems appropriate to present the alphabetized list, to which has been added information deemed pertinent, at the end of this article, rather than fragmented in the separate articles concerning the origin and specific properties of the species and varieties of garnet.


“Adelaide ruby”                Red garnet from South Africa
“African jade”                Massive green grossular garnet
“Alabandine ruby”                Almandine garnet
Allochroite                Andradite garnet from Switzerland (colorless)
“American ruby”                Pyrope garnet; also rose quartz
Aplome                Dark-brown, yellowish-green, or brownish-green andradite garnet (contains manganese)
“Arizona ruby”                Pyrope garnet
“Arizona spinel”                Garnet
“Australian ruby”                Garnet
Black garnet                Andradite garnet
Blythite                Manganic manganese garnet
Bobrowka garnet                Demantoid-alluvials Nizhniy Tagil in Urals; parent rock banks Bobrovsk, Sysertsk region
“Bohemian ruby”                Pyrope garnet; also rose quartz
Bredbergite                Magnesium andradite garnet from Sweden
Calderite                Manganous manganese-ferric iron garnet [Mn3Fe2(SiO4)3]
“California ruby”                Garnet
“Cape ruby”                Pyrope garnet (implies Cape of Good Hope or South African origin)
“Ceylonese ruby”                Almandine garnet
Cinnamon stone                Hessonite garnet
Colophonite                Resinous coarse cloudy yellow-brown andradite garnet; also non-gem variety vesuvianite
“Colorado ruby”                Pyrope garnet
“Elie or ely ruby”                Pyrope garnet (implies Isle of Ely or Scotland origin)
Emildine or emilite                Spessartite garnet from South Africa
Fashoda garnet                Pyrope garnet
“Fashoda ruby”                Pyrope garnet
“Garnet jade”                Massive green grossular garnet
Garnetoid                Hydrogrossular garnet
Goldmanite                Vanadiferous andradite garnet [Ca3V2Si3O12] crystals tiny, dark green
Gooseberry stone                Grossular garnet
“Green garnet”                Enstatite
Guarnaccino                 Yellowish-red garnet
Haplome                Andradite garnet (contains manganese); syn. Aplome
“Hematite garnet”                Synthetic iron-rich garnet
Hyacinth                Hessonite garnet; also zircon
Hydropyrope                A synthetic garnet
Hydrospessartine                A possible synthetic garnet
Jacinth                Hessonite garnet; also zircon
“Kandy spinel”                Almandine garnet
Kelyphite                Pyrope garnet inside chlorite
Kimseyite                Calcium-zirconium garnet from Arkansas [Ca3(Zr,Ti)2(Al,Si)3O12]
Knorringite                A chromiferous garnet [Mg3Cr2Si3O12]
Kollin garnet                Almandine garnet
Landerite                Pink grossular garnet from Mexico
Leuco-garnet                White garnet from Bohemia
Majorite                Purple garnet [Mg3(Fe,Al,Si)2Si3O12] found in a meteorite
“Montana ruby”                Red garnet
“Mountain ruby”                Red garnet
“Olivene”/“olevene”                Demantoid garnet
“Oregon jade”                Grossular garnet
Partschinite                Spessartite garnet from Rumania
Polyadelphite                Brownish-yellow andradite from New Jersey
Pyreneite                Grayish-black andradite garnet from France
Rhodolite                Intermediate pink, rose, or purplish to violet-red garnet between almandine and pyrope
“Rock ruby”                Pyrope garnet
“Rocky Mountain ruby”                Pyrope garnet
Roddingite                 Hydrogrossular garnet from New Zealand
Romanzonite                Dark-brown grossular garnet
Rose garnet                Pink grossular garnet; also rhodolite
Roselite or rosolite                Pink grossular garnet from Mexico
Rothoffite                Brown andradite garnet from Sweden
Schorlomite                Dark-brown to black andradite garnet [Ca3Ti2Fe3O12]
“Siberian chrysolite”                Demantoid garnet
Skiagite                Ferrous-ferric iron garnet from Scotland
“South African jade”                Massive green grossular garnet
Spalmandite                Intermediate almandine-pyrope garnet
Succinite                Amber-colored grossular garnet from Italy; also amber
Suriam garnet                Violet-red almandine garnet
Syriam/Syrian garnet                Violet-red almandine garnet
Topazolite                Greenish-yellow to yellow andradite garnet
Transvaal jade                Massive green grossularite from South Africa
Vermeille or vermeille garnet                Brownish-red almandine garnet; also pyrope garnet
“Vesuvian garnet”                Early name for leucite whose crystal form resembles that of garnet
“White garnet”                Translucent variety grossular garnet resembles white jade in appearance; also Leucite
Wiluite                Green variety of grossular garnet; also a greenish variety of vesuvianite
Xalastocite                Pink grossular garnet from Mexico
Yamatoite                Manganese vanadium silicate (Mn3V2Si3O12)