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Garnet

The garnet group of minerals show crystals with a habit of rhombic dodecahedrons and trapezohedrons. They are nesosilicates with the same general formula, A3B2(SiO4)3. The chemical elements in garnet include calcium, magnesium, aluminium, iron2+, iron3+, chromium, manganese, and titanium. Garnets show no cleavage and a dodecahedral parting. Fracture is conchoidal to uneven; some varieties are very tough and are valuable for abrasive purposes. Hardness is 6.5 - 7.5, specific gravity is 3.1 - 4.3, luster is vitreous to resinous, and they can be transparent to opaque.

The name "garnet" comes from the Latin granatus, a grain possibly in reference to malum garanatum (pomegranate) a plant with red seeds similar in shape, size and color to some garnet crystals.

There is a misconception that garnets are only a red gem but in fact they come in a variety of colors including purple, red, orange, yellow, green, brown, black, or colorless. The lack of a blue garnet was remedied in 1990's following the discovery of color-change blue to red/pink material in Bekily, Madagascar but these stones are very rare. Color-change garnets are by far the rarest garnets except uvarovite, which does not come in cuttable sizes. In daylight, their color can be shades of green, beige, brown, gray and rarely blue, to a reddish or purplish/pink color in incandescent light. By composition, these garnets are a mix of spessartine and pyrope, as are Malaya garnets. The color change of these new garnets is often more intense and more dramatic than the color change of top quality Alexandrite which is frequently disappointing, but still sells for many thousands of dollars (US) per carat. It is expected that blue color-change garnets will match Alexandrite prices or even exceed them as the color change is often better and these garnets are much rarer. The blue color-change type is mainly caused by relatively high amounts of vanadium (about 1 wt.% V2O3).

Six common varieties of garnet are recognized based on their chemical composition. They are pyrope, almandine or carbuncle, spessartite, grossularite (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series; 1. pyrope-almandine-spessarite and 2. uvarovite-grossularite-andradite.

Garnet is the birthstone for January, and has been used since the Bronze Age.

Garnet group endmembers

Pyralspite garnets - Al in B site

  • Almandine: Fe3Al2(SiO4)3
  • Pyrope: Mg3Al2(SiO4)3
  • Spessartine: Mn3Al2(SiO4)3

Almandite

Almandite, sometimes called almandine, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "little spark." The name Almandite is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandite is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandite occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandite has nicknames of Oriental garnet, almandine ruby, and carbuncle.

Pyrope

Pyrope, from the Latin pyropos, means similar to fire. It is ruby-red in color and chemically a magnesium aluminium silicate with the formula Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to almost black. Transparent pyropes are used as gemstones.

A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, from the Greek meaning "a rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the proportion of two parts pyrope to one part almandite. Pyrope has nicknames of Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian garnet from the Czech Republic. Another intriguing find is the blue color-change garnets from Madagascar, a pyrope spessatine mix. The color of these blue garnets is not like sapphire blue in subdued daylight but more reminiscent of the grayish blues and greenish blues sometimes seen in spinel However in white LED light the color is equal to the best corn flower blue sapphire or D block tanzanite this is due to the blue garnets ability to absorb the yellow component of the emitted light.

Pyrope is an indicator mineral for high pressure rocks. The garnets from mantle derived rocks, peridotites and eclogites, commonly contain a pyrope variety.

Spessartite

Spessartite or spessartine is manganese aluminium garnet, Mn3Al2(SiO4)3. Its name is derived from Spessart in Bavaria. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartite of a beautiful orange-yellow is found in Madagascar. Violet-red spessartites are found in rhyolites in Colorado and Maine.

Ugrandite group - calcium in A site

  • Andradite: Ca3Fe2(SiO4)3
  • Grossular: Ca3Al2(SiO4)3
  • Uvarovite: Ca3Cr2(SiO4)3

Andradite

Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black. The recognized subvarieties are topazolite (yellow or green), demantoid (green) and melantite (black). Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the Urals" from its occurrence there, and is one of the most prized of garnet varieties. Topazolite is a golden yellow variety and melanite is a black variety.

Grossularite

Grossular on display at the National Museum of Natural History.

Grossularite is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossularite is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossularite is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called tsavorite. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca3Cr2(SiO4)3. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia and Outukompu, Finland.

Less common species

  • Calcium in A site
    • Goldmanite: Ca3V2(SiO4)3
    • Kimzeyite: Ca3(Zr,Ti)2[(Si,Al,Fe3+)O4]3
    • Morimotoite: Ca3Ti4+Fe2+(SiO4)3
    • Schorlomite: Ca3(Ti4+,Fe3+)2[(Si,Ti)O4]3
  • Hydroxide bearing - calcium in A site
    • Hydrogrossular: Ca3Al2(SiO4)3-x(OH)4x
      • Hibschite: Ca3Al2(SiO4)3-x(OH)4x (where x is between 0.2 and 1.5)
      • Katoite: Ca3Al2(SiO4)3-x(OH)4x (where x is greater than 1.5)
  • Magnesium or manganese in A site
    • Knorringite: Mg3Cr2(SiO4)3
    • Majorite: Mg3(Fe,Al,Si)2(SiO4)3
    • Calderite: Mn3Fe3+2(SiO4)3

Knorringite

Knorringite is a magnesium chromium garnet with the formula Mg3Cr2(SiO4)3. Pure knorringite never occurs in nature. Garnet with high knorringite content may be generated only under high pressure. Knorringite is basically pyrope with a very high chromium content and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.

Synthetic Garnets

In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behaviour. By substituting specific sites with rare earth elements, for example, interesting magnetic properties can be obtained.

One example for this is gadolinium gallium garnet, Gd3Ga2(GaO4)3, which is synthesized for use in magnetic bubble memory.

Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstone. When doped with neodymium (Nd3+), these YAl-Garnets are useful as the lasing medium in lasers.

Uses of garnets

Pure crystals of garnet are used as gemstones. Garnet sand is a good abrasive. Pyrope varieties are used as kimberlite indicator minerals in diamond prospecting.

Garnets are very abundant in the lower crust and mantle and thus play an important role in geochemical understanding of the Earth.

The garnet is the official mineral and color of Bates College.

References

  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0471805807
  • Color Encyclopedia of Gemstones ISBN 0442203330
  • Mindat.org
  • Minerals.net
  • Mineral galleries
  • USGS Garnet locations - USA
  • Mineral Miners Garnet Info
  • Lets Talk Gemstones by Edna B. Anthony, Gemologist

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The garnet is the official mineral and color of Bates College. Hamilton is the surname of a renowned family from the Scottish Lowlands that has given its name to the town of Hamilton, South Lanarkshire, the Dukedom of Hamilton, and many people and places, the largest of which is the Canadian city of Hamilton, Ontario. Garnets are very abundant in the lower crust and mantle and thus play an important role in geochemical understanding of the Earth. . Pyrope varieties are used as kimberlite indicator minerals in diamond prospecting. Hamilton (Schooner) Schooner class ship during the War of 1812. Garnet sand is a good abrasive. Hamilton (movie), a Swedish-produced movie based on the character Carl Hamilton.

Pure crystals of garnet are used as gemstones. Hamilton Watch Company. When doped with neodymium (Nd3+), these YAl-Garnets are useful as the lasing medium in lasers. Hamilton Boys' High School, New Zealand. Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstone. Hamilton College, New York, USA. One example for this is gadolinium gallium garnet, Gd3Ga2(GaO4)3, which is synthesized for use in magnetic bubble memory. Hamilton, New South Wales, Australia.

By substituting specific sites with rare earth elements, for example, interesting magnetic properties can be obtained. Hamilton Island, Queensland, Australia. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behaviour. Hamilton, Victoria, Victoria, Australia. In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. Hamilton, Tasmania, Tasmania, Australia. It is used as an indicator mineral in the search for diamonds. Hamilton, Ontario (township), Canada (unrelated township).

Knorringite is basically pyrope with a very high chromium content and is often found in kimberlites. Hamilton, Ontario, Canada. Garnet with high knorringite content may be generated only under high pressure. Hamilton, New Zealand. Pure knorringite never occurs in nature. Hamilton, Leicester, England. Knorringite is a magnesium chromium garnet with the formula Mg3Cr2(SiO4)3. Hamilton Parish, Bermuda.

It is found in crystalline marbles and schists in the Ural mountains of Russia and Outukompu, Finland. Hamilton, Bermuda. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. Hamilton, South Lanarkshire, Scotland, the original one and location of Hamilton Palace. Uvarovite is a calcium chromium garnet with the formula Ca3Cr2(SiO4)3. William Hamilton Township, Hyde County, South Dakota. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name. Hamilton Township, Marshall County, South Dakota.

One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called tsavorite. Hamilton Township, Charles Mix County, South Dakota. Grossularite is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite. South Dakota

    . Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Hamilton Township, Tioga County, Pennsylvania. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Hamilton Township, Monroe County, Pennsylvania.

    The name grossularite is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Hamilton Township, McKean County, Pennsylvania. Grossularite is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. Hamilton Township, Franklin County, Pennsylvania. Topazolite is a golden yellow variety and melanite is a black variety. Hamilton Township, Adams County, Pennsylvania. Demantoid has been called the "emerald of the Urals" from its occurrence there, and is one of the most prized of garnet varieties. Pennsylvania

      .

      Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Hamilton Township, Warren County, Ohio. The recognized subvarieties are topazolite (yellow or green), demantoid (green) and melantite (black). Hamilton Township, Lawrence County, Ohio. Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black. Hamilton Township, Jackson County, Ohio. Violet-red spessartites are found in rhyolites in Colorado and Maine. Hamilton Township, Franklin County, Ohio.

      Spessartite of a beautiful orange-yellow is found in Madagascar. Ohio

        . It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Hamilton Township, North Dakota. Its name is derived from Spessart in Bavaria. North Dakota
          . Spessartite or spessartine is manganese aluminium garnet, Mn3Al2(SiO4)3. Hamilton Township, North Carolina.

          The garnets from mantle derived rocks, peridotites and eclogites, commonly contain a pyrope variety. North Carolina

            . Pyrope is an indicator mineral for high pressure rocks. Hamilton Township, Mercer County, New Jersey. The color of these blue garnets is not like sapphire blue in subdued daylight but more reminiscent of the grayish blues and greenish blues sometimes seen in spinel However in white LED light the color is equal to the best corn flower blue sapphire or D block tanzanite this is due to the blue garnets ability to absorb the yellow component of the emitted light. Hamilton Township, Atlantic County, New Jersey. Another intriguing find is the blue color-change garnets from Madagascar, a pyrope spessatine mix. New Jersey
              .

              Pyrope has nicknames of Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian garnet from the Czech Republic. Hamilton Township, Nebraska. A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, from the Greek meaning "a rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the proportion of two parts pyrope to one part almandite. Nebraska

                . Transparent pyropes are used as gemstones. Hamilton Township, Harrison County, Missouri. The color of pyrope varies from deep red to almost black. Hamilton Township, Caldwell County, Missouri.

                It is ruby-red in color and chemically a magnesium aluminium silicate with the formula Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous iron. Missouri

                  . Pyrope, from the Latin pyropos, means similar to fire. Hamilton Township, Van Buren County, Michigan. Almandite has nicknames of Oriental garnet, almandine ruby, and carbuncle. Hamilton Township, Gratiot County, Michigan. Almandite occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Hamilton Township, Clare County, Michigan.

                  Chemically, almandite is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Michigan

                    . The term "carbuncle" is derived from the Latin meaning "little spark." The name Almandite is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Hamilton Township, Lee County, Illinois. Almandite, sometimes called almandine, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). Illinois
                      . . Hamilton Township, Hamilton County, Iowa.

                      Garnet is the birthstone for January, and has been used since the Bronze Age. Hamilton Township, Franklin County, Iowa. uvarovite-grossularite-andradite. Hamilton Township, Decatur County, Iowa. pyrope-almandine-spessarite and 2. Iowa

                        . The garnets make up two solid solution series; 1. Hamilton Township, Sullivan County, Indiana.

                        They are pyrope, almandine or carbuncle, spessartite, grossularite (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. Hamilton Township, Jackson County, Indiana. Six common varieties of garnet are recognized based on their chemical composition. Hamilton Township, Delaware County, Indiana. The blue color-change type is mainly caused by relatively high amounts of vanadium (about 1 wt.% V2O3). Indiana

                          . It is expected that blue color-change garnets will match Alexandrite prices or even exceed them as the color change is often better and these garnets are much rarer. Hamilton Township, Arkansas.

                          The color change of these new garnets is often more intense and more dramatic than the color change of top quality Alexandrite which is frequently disappointing, but still sells for many thousands of dollars (US) per carat. Arkansas

                            . By composition, these garnets are a mix of spessartine and pyrope, as are Malaya garnets. Hamilton County, Texas. In daylight, their color can be shades of green, beige, brown, gray and rarely blue, to a reddish or purplish/pink color in incandescent light. Hamilton County, Tennessee. Color-change garnets are by far the rarest garnets except uvarovite, which does not come in cuttable sizes. Hamilton County, Ohio.

                            The lack of a blue garnet was remedied in 1990's following the discovery of color-change blue to red/pink material in Bekily, Madagascar but these stones are very rare. Hamilton County, New York. There is a misconception that garnets are only a red gem but in fact they come in a variety of colors including purple, red, orange, yellow, green, brown, black, or colorless. Hamilton County, Nebraska. The name "garnet" comes from the Latin granatus, a grain possibly in reference to malum garanatum (pomegranate) a plant with red seeds similar in shape, size and color to some garnet crystals. Hamilton County, Kansas. Hardness is 6.5 - 7.5, specific gravity is 3.1 - 4.3, luster is vitreous to resinous, and they can be transparent to opaque. Hamilton County, Iowa.

                            Fracture is conchoidal to uneven; some varieties are very tough and are valuable for abrasive purposes. Hamilton County, Indiana. Garnets show no cleavage and a dodecahedral parting. Hamilton County, Illinois. The chemical elements in garnet include calcium, magnesium, aluminium, iron2+, iron3+, chromium, manganese, and titanium. Hamilton County, Florida. They are nesosilicates with the same general formula, A3B2(SiO4)3. Hamilton City, California.

                            The garnet group of minerals show crystals with a habit of rhombic dodecahedrons and trapezohedrons. Hamilton, Wisconsin. Anthony, Gemologist. Hamilton, Washington. Lets Talk Gemstones by Edna B. Hamilton, Virginia. Mineral Miners Garnet Info. Hamilton, Texas.

                            USGS Garnet locations - USA. Hamilton, Ohio. Mineral galleries. Hamilton, North Dakota. Minerals.net. Hamilton, North Carolina. Mindat.org. Hamilton (town), New York.

                            Color Encyclopedia of Gemstones ISBN 0442203330. Hamilton (village), New York. Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0471805807. Hamilton, Nevada. Calderite: Mn3Fe3+2(SiO4)3. Hamilton, Montana. Majorite: Mg3(Fe,Al,Si)2(SiO4)3. Hamilton, Missouri.

                            Knorringite: Mg3Cr2(SiO4)3. Hamilton, Mississippi. Magnesium or manganese in A site

                              . Hamilton, Michigan. Katoite: Ca3Al2(SiO4)3-x(OH)4x (where x is greater than 1.5). Hamilton, Massachusetts. Hibschite: Ca3Al2(SiO4)3-x(OH)4x (where x is between 0.2 and 1.5). Hamilton, Kansas.

                              Hydrogrossular: Ca3Al2(SiO4)3-x(OH)4x

                                . Hamilton, Iowa. Hydroxide bearing - calcium in A site
                                  . Hamilton, Illinois. Schorlomite: Ca3(Ti4+,Fe3+)2[(Si,Ti)O4]3. Hamilton, Indiana. Morimotoite: Ca3Ti4+Fe2+(SiO4)3. Hamilton, Georgia.

                                  Kimzeyite: Ca3(Zr,Ti)2[(Si,Al,Fe3+)O4]3. Hamilton, Alabama. Goldmanite: Ca3V2(SiO4)3. Professor Hamilton, a supporting character from Superman comics. Calcium in A site

                                    . Marcus Hamilton, a character from the TV series Angel. Uvarovite: Ca3Cr2(SiO4)3. Carl Hamilton, the fictional spy created by Jan Guillou.

                                    Grossular: Ca3Al2(SiO4)3. William Hamilton - many people, see Disambiguation Page. Andradite: Ca3Fe2(SiO4)3. Walter Kerr Hamilton, English bishop. Spessartine: Mn3Al2(SiO4)3. Tyler Hamilton, American cyclist. Pyrope: Mg3Al2(SiO4)3. Thomas Hamilton - many people, see Disambiguation Page.

                                    Almandine: Fe3Al2(SiO4)3. Terrick Hamilton, Scottish linguist. Richard Hamilton - many people, see Disambiguation Page. Hamilton, science fiction author. Peter F.

                                    Patrick Hamilton - many people, see Disambiguation Page. Hamilton, Vice Chair of the 9/11 Commission. Lee H. Hamilton, American horror and fantasy writer.

                                    Laurell K. Jim Hamilton - Scottish footballer. Keith Hamilton - former defensive tackle for the New York Giants. James Hamilton - many people, see Disambiguation Page.

                                    Ian Hamilton - many people, see Disambiguation Page. Henry Hamilton, British general. Hamilton, of the band British Sea Power. Guy Hamilton, British film director.

                                    George Hamilton - many people, see Disambiguation Page. Francis Hamilton, Scottish physician and geographer of the Bengal region. Lady Emma Hamilton, mistress of Lord Nelson. Elizabeth Hamilton, Scottish writer.

                                    Edmond Hamilton, science fiction writer. Edith Hamilton, writer on mythology. Eamon Hamilton, of the bands British Sea Power and Brakes. Donald Hamilton - American writer.

                                    David Hamilton - many people, see Disambiguation Page. Charles Hamilton - many people, see Disambiguation Page. Billy Hamilton, baseball player. Bethany Hamilton, surfer.

                                    Anthony Hamilton - many people, see Disambiguation Page. Andrew Hamilton - many people, see Disambiguation Page. Alice Hamilton, American toxicologist. Alexander Hamilton, American statesman.