Green Grossular Garnets
Gem dealers also commonly apply their own trade names based on the location of Garnet mining deposits, such as red "Mozambique" Garnet (determined by our investigation to be Standard Pyrope), red "Tanga" Garnet from Tanzania (determined to be Malaya), orangey red "Umbalite" from Tanzania (determined to be Malaya), brownish pink "Maralambo" Garnet from Madagascar (determined to be Pastel Pyrope), JTV's brownish pink "Massasi Bordeaux" Garnet from Tanzania (determined to be Malaya), pinkish orange "Mahenge" Garnet from Tanzania (determined to be Malaya) and orange Tangawizi Garnet from Tanzania (Malaya). The term "Malaya" itself is a trade name/variety name for any Pyralspite Garnet with unusual color and appearance.
Predicting Garnet Color: The Hoover method primarily measures iron and manganese in Garnets to determine the proportions of 2 or 3 end member Garnet components. The position of a graph point on a Hoover diagram predicts color in Almandine and Uvarovite, but the other 4 Garnet species show much more variation in color. Low concentrations of chromophores such as chromium, vanadium and charge transfer processes affect color but contribute little or nothing to magnetic susceptibility measurements.
Green Grossular Garnet is an example. The lowest magnetic susceptibility we can measure in green Grossular Garnets (under 20 SI) may have little if anything to do with the coloring agents chromium and vanadium from Uvarovite/Knorringite/Goldmanite. The magnetism we detect is likely due to entirely to iron (Fe3+) from the Andradite end-member.
Because the positioning of Garnet graph points on a Hoover diagram shows only the major components of a gem, it is not possible to accurately predict variations in Garnet color and color intensity by the location of those graph points when color is influenced by minor components. Color variations may be due to small variations in chromium/vanadium content, and in some cases possibly to color processes such as intervalence charge transfer and even color centers. Two Pyralspite Garnets of entirely different color can have the same proportions of the 3 end members (Pyrope, Almandine & Spesssartine) and share the same graph point location, as illustrated by the 2 gems below.
Color Change Garnets
"Mozambique" Garnet (Pyrope)
As purely descriptive terms, hybrid names convey useful information about the extent of a Garnet's hybridization. But when detailed information about any individual Garnet's composition can readily be determined using the RIMS method, we don't need to manufacture new intermediate species names in order to classify Garnets. We can simply use the standard mineralogical classification system, which classifies an individual specimen by its primary end-member component (i.e. the species with the highest percentage composition). Our RIMS graph method determines the primary species by the trisection in which a gem's graph point falls. A Malaya Garnet known to contain 40% Pyrope, 35% Almandine and 25% Spessartine is simply classified as a Malaya variety of the Pyrope species.
Readers can click on Gem Garnet Classification to view a proposed Garnet classification system developed during this study. For Garnet species and varieties that overlap in both composition and color, there are no clear classification boundaries, but still we can attempt to make meaningful separations (see page 13 for more details).
"Champagne" Garnet (Andradite)
The olive-brown Garnet from Kenya and the purple Garnet from Bekily, Madagascar pictured above are both Color Change Pyropes, or if you prefer, Color Change Pyrope-Spessartines. Both gems change to pink color in incandescent light. They have nearly identical refractive indexes and magnetic susceptibilities. Both have the same major composition of 51% Pyrope and 49% Spessartine. But a slightly higher concentration of chromium/vanadium (which is magnetically undetectable) appears to be responsible for purple color in the oval gem. The shared graph point of these 2 gems is shown below, along with a spectrometer graph showing higher chromium/vanadium content in the purple Garnet.
Color Change Garnets with Identical Primary Composition
Garnets of Different Color Can Have the Same Primary Composition
As examples of mixing in small percentages, Green Grossulars such as Tsavorite and Merelani contain only about 5% Andradite (iron, Fe3+). Ions of vanadium (V3+) and chromium (Cr3+), which impart most of the bright green color found in these Grossulars, are derived from even slighter mixing with Goldmanite (V3+), Uvarovite (Cr3+) and Knorringite (Cr3+) Garnet species. That is to say, vanadium and chromium ions partially replace aluminum in the B site of the chemical formula for Grossular gems. The vanadium/chromium oxides that give green Grossulars most of their color make up less than 2% of the gem's weight, and can be as low as 0.1% or less.
© Kirk Feral 2011, All Rights Reserved. These materials may be duplicated for educational purposes only. No part of this website may be duplicated or distributed for profit, for commercial purposes, or for posting to another website without the expressed written consent of the copyright holder.
Idiochromatic or Allochromatic?
Pyrope and Grossular are allochromatic Garnet species (colored by impurities, colorless when pure), while Almandine, Spessartine, Andradite and Uvarovite are considered idiochromatic (self-colored, never completely colorless). The strong colors and magnetic responses seen in allochromatic Pyrope and Grossular are due to mixing with the idiochromatic Garnet species. In reality, every Garnet gem (except perhaps some red Spessartine gems) is a mixture of idiochromatic and allochromatic species components. The amount of mixing can sometimes be very small.
When we examine Garnets with a spectrometer, we see that chromium/vanadium is important to color in all varieties of Pyrope Garnet, with small amounts resulting in red, pink, purple, blue and green colors. Among Garnets containing Spessartine, manganese (Mn2+) primarily controls orange and yellow colors. In Garnets with Almandine content, iron (Fe2+) contributes purplish red color.
Andradite Garnet colors such as brown, red and yellow are likely due to Fe2+-Ti4+ and/or Fe2+-Fe3+ charge transfer processes, while yellowish green color in Andradite is the result of iron (Fe3+), and pure green or blue-green is due to chromium/vanadium.
Among Grossular Garnets, rich green colors are primarily the result of vanadium/chromium, although iron (Fe3+) can contribute yellowish green color. Orange colors in Grossular Garnet are caused primarily by traces of manganese (Mn2+) and possibly charge transfer involving manganese, while pink colors are caused by trivalent manganese (Mn3+). Iron (Fe3+) is often colorless (cryptic) in Grossulars, although charge transfer involving iron may contribute to red and orange color. More research is needed to provide a more complete picture of color in Garnets, especially regarding charge transfer processes..
Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Causes of Color
We know that a Garnet's color cannot always be attributed to the chromophores present in its primary end member. The orange of Spessartine (Mn2+) and red of Almandine (Fe2+) can be attributed to their primary species content, but the red of Pyrope Garnet is not due to Pyrope content (Mg2+ magnesium). The red is derived from Almandine (Fe2+ iron) and Uvarovite/Knorringite/Goldmanite (Cr3+/V3+ chromium/vanadium).
Very small percentages of chromophores from other Garnet end members can dramatically affect color. As another example, the color of blue Color Change Garnet is not derived from it's primary component Pyrope (Mg2+ magnesium), but rather from Spessartine (Mn2+ manganese) in combination with minor Goldmanite (V3+ vanadium) and Uvarovite (Cr3+ chromium) content (Schmetzer et. al. 2009).
Garnet Names Based on Chemical Composition
Naming Garnets based on color is often a bad idea, as many different species and varieties of Garnet have the same body color. Gemologists mostly rely on chemical composition to classify Garnets, although the composition for any individual Garnet gem is generally not known. Even when composition is known, there is no consensus on the most appropriate naming or classification system. At times gemologists simply ignore differences in composition between 2 gems in favor of common color, or common color phenomena such as color change, when naming Garnets.
For example, purple Almandines are sometimes referred to as Rhodolites, even though Almandine is a different species whose chemical composition can be very different from that of the reddish purple Pyropes (Rhodolites) of similar color. Gemologists also typically classify all Garnets that show color change as Color Change Garnets, failing to make any distinction as to whether the Color Change Garnet is primarily Pyrope or primarily Spessartine. We feel that using color or color phenomena as the primary parameter for classifying Garnets is a practice that perpetuates misunderstanding. Gems of different species can appear identical, and color change can occur in species and varieties other than typical Color Change Garnets.
When attempting to name Garnets based on composition, some gemologists (Manson/Stockton, Hanneman) have created new species names that represent hybrid Garnets. Manson/Stockton proposed that we recognize Pyrope-Almandine and Pyrope-Spessartine as additional species of Garnets that are intermediate in composition between the two end-member species Pyrope and Spessartine. These Garnets would have graph points located along the mid-section of the joining line between two end members. GIA now regards these two hybrids as distinct species in addition to the 6 primary species of gem Garnets.
51% Pyrope, 49% Almandine
34% Pyrope, 66% Spessartine
A Rainbow of Color in Garnets
Garnet Names Based on Color
The mixing of Garnets in infinite combinations makes the naming of Garnets species and varieties difficult. In the marketplace, it is a free-for-all when it comes to trade names. Most trade names are based on peculiarities of color. Gem dealers want to appeal to buyers by using familiar and colorful names like "Cranberry" Rhodolite (purple-red Rhodolite), "Champagne" Garnet (yellow-brown Andradite), and "Mandarin" Garnet (dark orange Spessartine or dark orange Hessonite).
Confusion arises when dealers fail to provide the appropriate variety names or even species names for Garnets that are given these trade names. Often dealers don't know the correct identification. JTV does know, but it takes extraordinary liberties with nomenclature, trademarking many new names for Garnets such as Mpito Color Change Garnet from Kenya, VermehloGarnet (Pyrope-Amlmandine) from Mozambique, and Serengeti Spessartite Garnet from Tanzania, all touted as new and unusual discoveries. Such trade names are generally not used by gemologists, but a few trade names such as Tsavorite, Rhodolite, Malaya and Mali Garnet have become widely accepted as variety names.
"Umbalite" Garnet (Malaya)