The garnets constitute a group of minerals which share a common structure. Common end member compositions are:

• Almandine ---------- Fe²⁺₃Al₂(SiO₄)₃
• Andradite ----------- Ca₃Fe³⁺₂(SiO₄)₃
• Grossular ----------- Ca₃Al₂(SiO₄)₃
• Pyrope -------------- Mg₃Al₂(SiO₄)₃
• Spessartite ---------- Mn₃Al₂(SiO₄)₃
• Uvarovite ----------- Ca₃Cr₂(SiO₄)₃

As would be expected from a chemically complex group of minerals, garnets owe their color to a variety of causes. Absorption by Cr³⁺, V³⁺, Fe³⁺ (6-coordinated site), Fe²⁺ (8-coordinated site), Mn²⁺, and Fe²⁺ - Ti⁴⁺ intervalence charge transfer can all be found in the spectra of garnets.

Spectra of representative garnets

Colorless garnets

• Picture of a colorless grossular from Merelani Hills, Tanzania.              

Dark red to pink garnets with Fe²⁺

Almandine GRR 892, India. 1.0 mm thick

Almandine GRR 1040, North River Garnet Company, Warren Co., NY, [Fe_1.35 Mg_1.30 Ca_0.26 Mn_0.03] Al_1.98(SiO₄)₃; 1.54 mm thick; data file

Almandine, GRR 1056, Wrangell, Alaska, [Fe_2.03Mg_0.67Ca_0.18Mn_0.07]Al_2.0(SiO₄)₃, colored light reddish-purple by Fe²⁺ , normalized to 1.0 mm thickness; data file

Almandine, GRR 1776, dark reddish-purple gemstone, [Fe_2.5Mg_0.3Mn_0.2Ca_0.1]Al_2.0(SiO₄)₃, probably from India, Graphed at 1.0 mm, normalized from 0.213 mm thickness. Visible region only; data file

Pyrope, GRR 83, rhodolite variety, Umba River Valley, Tanzania, colored pink by Fe²⁺; 0.78 mm thick;  data file

Pyrope, GRR 131, unknown locality, probably Tanzania, colored pink by Fe²⁺ ; Approximate 8-coordinated site composition: Mg_1.82 Fe_0.99 Ca_0.04 Mn_0.04. Plotted as 2.007 mm thick; data file

Pyrope, GRR 1121i, Wesselton, South Africa, Mg_2.32 Mn_0.01 Fe_0.43 Ca_0.27 Al_1.95 Cr_0.02, 1.693 mm thick; data file

Pyrope, GRR 1121f, Wesselton kimberlite diamond mine, South Africa. Mg_2.09 Mn_0.02 Fe_0.67 Ca_0.26 Al_1.89 Cr_0.03 Ti_0.03. Plotted as 0.547 mm thick; data file; data file

Pyrope-spessartine, GRR 1267, Kenya, sold commercially as "Malaya Garnets". 1.686 mm thick; data file

Pyrope, Mon-9, Monastery Mine, South Africa, 1.37 mm thick; data file . Showing the three near-IR Fe²⁺ bands in the 1000 - 2500 nm range and weaker broad OH bands that are visible in the 2800 nm region.

Orange garnets with Mn²⁺

• Spessartine, GRR 72, Rutherford Mine, VA, colored orange by Mn²⁺ 3.45 mm thick; data file
• Spessartine, GRR 43, Rutherford Mine, Amelia, VA, colored orange by Mn²⁺ , expanded view from a 2.165 mm thick sample. data file
• Andradite-spessartine, GRR 149, Franklin, NJ, colored orange due to the mixture of Fe, Mn and Ti. Approximate compositon: Ca_2.23, Mn_0.72, Fe_1.48, Al_0.44, Ti_0.02. Plotted as 1.0 mm thick.
  

Orange to black garnets with Fe²⁺, Fe³⁺ and Ti⁴⁺

• Andradite, GRR 568, variety (black when thick) melanite  , San Benito Mountains, California, with both Fe³⁺ and Ti⁴⁺, 0.102 mm thick. Approximate composition in the 6-coordinated site: Fe_1.16, Ti_0.74, Al_0.007. data file
• Andradite, GRR 568, variety melanite, core of the sample with a redidish-brown, almost purple color, 0.125 mm thick. Data file
• Andradite (variety melanite), GRR 755, Santa Rita Peak, San Benito Mountains, California. The garnet crystals  are black, but when thinned, it becomes dark brown-orange. The sharp peak near 441 nm is from Fe³⁺; approximate composition in the 6-coordinated site: Fe_1.69, Ti_0.27, Al_0.04.   0.045 mm thick, data file
• Grossular, GRR 53, (orange-brown) variety hessonite, . Jeffrey Mine, Asbestos (now Val-des-Sources), Québec, Canada
  
• Grossular, GRR 1113, (orange-brown), North Hill, Riverside, California. (Ca_2.86 Fe_0.12 Mn_0.02)  [Al_1.52 Fe_0.43 Ti_0.05], with both Fe²⁺ and Fe³⁺, 0.336 mm thick. data file
• Garnet, GRR3197, (orange) Tunduru, Tanzania , 2.084 mm thick. Data file.
  
• Grossular GRR 1386, (brown), Lalatema, Moshi, Kenya  , 4.126 mm thick. Data file

Yellow-green garnets with Fe³⁺

• Andradite, GRR 48, Val Malenco, Sondrio Province, Italy , colored greenish-yellow by Fe³⁺, nesr end-member comopsition. Plotted as 1.03 mm thick;  data file. 
• Andradite, GRR 1268, San Benito Mountains, California. Greenish-yellow crystals.
  
• Andradite, GRR 1765, Ural Mountains, Russia , colored by Fe³⁺, 0.737 mm thick;  data file. 
• Grossular, GRR 1934b from Sierra de las Cruces, near Lake Jaco, Chihuahua, Mexico, 2.60 mm thick. This is a yellow-green rim on a crystal with a small black core.  Data file
• Andradite, green GRR 2185 plotted as 2.38 mm thick and brownish GRR 2185, plotted as 2.38 mm thick, from Farm Tubussis, Erongo Moutains, Namibia.   These garnets vary from green to brownish. Data files: green;  brown
• Andradite, GRR 3137 from the Antetezajbato Mine, Ambanja, Madagascar,   plotted as 1.00 mm thick. This light green garnet has the spectrum with the least deviation from pure andradite. Data file
  

Green garnets with V³⁺ and Cr³⁺

• Uvarovite, GRR 1019, Jacksonville, California, colored green by Cr³⁺  . Approximate composition of the 6-coordiated site: Cr_1.19 Fe_0.57 Al_0.15 Ti_0.04 V_0.01. Plotted as 0.029 mm thick. data file
• Grossular, variety Tsavorite, GRR 229, Tanzania , colored green by 0.11 wt% V³⁺.  Plotted as 0.758 mm thick. data file
• Andradite, bright green variety demantoid, GIA 2952, Russia , 2.5 mm thick.The spectrum of the slab has both the features of Fe³⁺ and the features of Cr³⁺. Data file.  
• Andradite, variety demantoid, GRR 2666 & GRR 2667, Kladovka Mine, Ural Mountains, Russia. Slabs made from two 0.639 mm thick crystals; orange as mined  and green after heat-treatment  which is used to remove the brown component of color often found in the natural stones.  Data files: GRR 2666 -  orange; GRR 2667 - green

Red garnets with Cr³⁺

• Pyrope, GRR 332, Ugelvik, Oteroy, Norway. Upper mantle sample colored by Cr³⁺. Plotted as 0.80 mm thick.
• Red pyrope, GRR 1055, Moses Rock, Utah , Chromian-pyrope, Ca_0.33 Mn_0.02 Mg_2.13 Fe_0.54 Al_1.95 Cr_0.07, 0.840 mm thick; data file
  
  

Red and pink garnets with Mn³⁺

• Dark pink grossular GRR1890, from Sierra de Cruces Range, Coahuila, Mexico, 1.17 mm thick. These garnets are often called raspberry grossular . They contain about 1 to 1.5% manganese oxide. Only about 10 percent of the manganese, which is in the 3+ oxidation state, is responsible for the pink color.  Data File.

Blue to purple garnets

• Pyrope, GRR 2225, Tunduru, Tanzania. This is a color-change pyrope-spessartine garnet  that goes from pale purplish-blue to light purple depending upon the illumination. It contains about 0.6 wt% vanadium. Thickness: 2.426 mm. Data file.

Luminescence Spectra of Garnets

• Grossular, GRR 1386, 365 nm excitation, nearly colorless garnet   from the Merelani Hills, Tanzania . Data files;  RT,   LN₂
• _{Grossular, GRR 1893, 365 nm excitation, green Cr-containing crystal  from Asbestos, Ontario, Canada. Data file:  RT}
• _{Grossular, GRR 2067, 365 nm excitation, green tsavorite , from the Lelatema Mountains, Tanzania. Data files: RT,  -78 C}

Link to an extensive collection of references to mostly color and visible spectroscopy of garnets

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