Green Grossular Garnets
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. Gem dealers understandably want to appeal to buyers by using colorful descriptive names like Cranberry Rhodolite (purple-red Rhodolite), Champagne Garnet (yellow-brown Andradite), and Mandarin Garnet (dark orange Spessartine or dark orange Hessonite).
Gem dealers also commonly apply their own trade names based on the location of gem deposits, such as Mozambique Garnet (determined by this researcher's study to be Pyrope), Tanga Garnet from Tanzania (determined to be Malaya), and Massasi Bordeaux Garnet from Tanzania (determined to be Malaya). Dealers seldom provide the appropriate variety names for these Garnets. 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.
Graph of All Gem Garnets
The graph below is a summary of over 500 Garnets collected and tested by this researcher in an ongoing study conducted since 2010. This is the first time a diagram of the composition of all gem Garnet varieties, as determined by magnetic susceptibility and refractive index, has been published. A broad sampling of color variation is represented, and many are rare or unusual Garnets. Species and variety names are color-coded to match the color of their graph points. For the sake of clarity, the numerous Grossular color variety names are listed on the left side of the graph rather than at their graph points. You can zoom your page view to 125% or more to see details clearly. Due to overlapping graph points, not all 500 points on the graph are visible.
The Hoover method primarily measures iron and manganese in Garnets. Low concentrations of metals such as chromium and vanadium, which have low magnetic susceptibilities, probably cannot be detected by magnetic measurements. Grossular Garnet is an example. The low magnetic susceptibility we can measure in green Grossular Garnets (under 2 SI) is probably not due to the coloring agents chromium and vanadium. The magnetism we detect is likely due to entirely to iron from the Andradite end member.
For these reasons, it is not possible to accurately predict variation in color and intensity of color in Garnets by the location of graph points on the ternary. Two Garnets of entirely different color can share precisely the same graph point location, as exemplified by the 2 gems below.
Color Change Garnets
Graph of All Gem Garnets (over 500 Samples)
Even among gemologists there is no consensus on how Garnets should be named or classified. Some gemologists (Manson/Stockton, Hanneman) have created new species names that represent hybrids such as Pyrope-Almandine and Pyrope-Spessartine, which refer to Garnets that contain two major components. These would have graph points located along the mid-section of a boundary line between two end members. GIA now considers the above 2 hybrids as distinct species in addition to the 6 primary species.
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 by using the RIMS method, we don't need to create new 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 Garnet gem known to contain 40% Pyrope, 35% Almandine and 25% Spessartine is simply classified as a variety of Pyrope (in this case, Malaya).
Readers can click on Gem Garnet Classification to view an a proposed Garnet classification system developed during this study. For Garnet species and varieties that overlap in composition and color, there are no clear boundaries, but we can attempt to make meaningful separations (see page 6 for more details).
Color Change Pyrope
Color Change Spessartine
When we do the calculation for the Malaya Garnet graph point shown above, we find that this particular gem is 58% Pyrope, 22% Spessartine and 20% Almandine. Readers can do this simple calculation after measuring the above lines on their computer screen with a ruler.
A photograph of the Malaya Garnet represented by the above graph point is shown below, along with a representation of its composition. Percentages of possible other minor components such as Grossuar or Uvarovite have not been calculated and are not shown in this representation.
Percentages of the 3 major components of a Malaya Garnet
This researcher considers Malaya to be primarily a variety of Pyrope with unusual color: red, pinkish red, pink, brownish pink, orange, brownish orange, orangey red and even near-colorless. Gems are often lighter in tone than Standard Pyrope gems, and lighter Malayas often exhibit color-shift.
Three Lines Intersect the Malaya Garnet Graph Point
Orangey Red Malaya
Without any calculations, the aproximate composition of the Malaya Garnet is instantly known simply by looking at the position of its graph point in relation to the 3 end member species of the ternary. Positioned toward the Pyrope end member, and equidistant from both the Pyrope-Almandine and Pyrope-Spessartine boundary lines, we know this gem is predominantly Pyrope with smaller amounts of Almandine and Spessartine in roughly equal amounts.
Malaya Garnet Graph Point
Percentage of composition for each end member species is calculated by measuring the full length of a line, then the length of the segment furthest from the apex (between the graph point and where the line ends at the ternary boundary), and finally dividing the far segment length by the full line length. We do this 3 times, once for each line initiating from its end member apex. The 3 percentages of end members should add up to 100%.
Graph of Malaya Garnets (RI range 1.735- 1.779, SI range 12.17- 30.89)
The olive-brown Garnet from Kenya and the purple Garnet from Bekily, Madagascar are both Color Change Pyropes, or if you prefer, Color Change Pyrope-Spessartines. Both gems glow pink in incandescent light. They have identical refractive indexes and magnetic susceptibilities. Both have the identical major composition of 51% Pyrope and 49% Spessartine, but small variations in minor chemical content is responsible for the differences in the daylight color of these 2 gems. Their shared graph point is shown below.
Color Change Garnets with Identical Primary Composition
Garnets of Different Color Can Have the Same Primary Composition
As an example, Green Grossulars such as Tsavorite and Merelani contain only about 5% Andradite (with iron,Fe3+). Chromium (Cr3+) and vanadium (V3+) ions,
Pale Pink Malaya in Daylight
© 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.
Some varieties shown on the All Gem Garnet graph above have refractive index ranges that do not match exactly the ranges published by GIA. For example, the low RI for Rhodolite Garnet in this study is 1.738, while the lower limit given by GIA is 1.740. The high RI for Standard Pyrope in this study is 1.77, while the GIA high is 1.756 (Pyropes with higher RI's are classified by GIA as Pyrope-Spessartines). No Uvarovite graph points are shown on the graph, as druzy gems cannot be measured for RI or susceptibility by the methods used in this study.
The graph above clearly shows that Pyralspite Garnet species mix with each other much more freely than do Ugrandite Garnet species. Ugrandites are composed primarily of one end member or another, with not much in the way of intermediate examples. All Ugrandites tested in this study fall along the Grossular-Andradite boundary line (none along the Uvarovite-Andratide or Grossular-Uvarovite lines). No true Grossular-Andradites with intermediate composition have been found by this researcher, meaning no graph points fall further than a third of the away from the closest end member along the Grossular-Andradite boundary. Most gemologists currently refer to Mali Garnet as a Grossuar-Andradite variety, but Mali is more clearly characterized simply as a Grossular Garnet.
Malaya is a Variety of Pyrope
Pale pink Malayas, sometimes seen with a slight purple undertone, can show slight color shift by flashing to red in incandescent light. Pale pinks also show color change from pink in daylight to gold in fluorescent light. These are Pyrope-Spessartines with pink color due to a small amount of chromium.
Masasi Bordeaux Garnet™ (# 1)
Idiochromatic or Allochromatic? < 3 >
Most gem Garnet species are idiochromatic. Only Pyrope and Grossular Garnets, the 2 least magnetic species, are allochromatic. The strong colors and magnetic responses seen in Pyrope and Grossular are due to their mixing with idiochromatic Garnet species. In reality, every Garnet gem (except perhaps some red Spessartines) is a mixture of idiochromatic and allochromatic components. The amount of mixing can sometimes be very small.
Other details about cause of color in Garnet gems are not completely understood by gemologists. Selective light absorption by the various metal ions found in Garnets may not be the only thing causing color. The phenomenon of intervalence charge transfer between ferrous and ferric iron ions, and between iron and titanium ions, might also be involved. More research in this area is needed to provide a more complete picture of color in Garnets.
Based on the location of graph points in this study, this researcher considers Standard Pyrope, Rhodolite, Chrome Pyrope, Pastel Pyrope and Malaya to be varieties of the Pyrope species. Color Change Garnet can be classified as either a variety of Pyrope or Spessartine, depending on a particular gem's composition. This researcher does not consider Color Change Garnet to be a sub-variety of Malaya (as do some gemologists). Study results show that Malaya is a variety of Pyrope in the vast majority of cases. There is considerable overlap in composition among different varieties, particularly between Rhodolite, Malaya and Standard Pyrope, and discriminating between these can at times be somewhat subjective (see page 6).
Determining Percentages of 3 End Member Species
More often than not, graph points of gems fall inside a ternary rather than along a line connecting two end members. This is particularly noticeable with Rhodolite (purple graph points) and Malaya (yellow graph points) shown in the All Garnet graph above. Placement toward the center of the ternary indicates that all 3 end-members of the ternary are major components of the gem. A graph point for a Malaya Garnet is shown below near the center of the Pyrope trisection of the Pyralspite ternary.
The exact percentages of each end member species can be easily calculated. We simply draw a straight line that starts at an end member apex, passes through the graph point, and ends at the opposite boundary. We do this 3 times, one line from each of the 3 end member apexes.
The Tanzanian Garnet trademarked in 2011 by Jewelry Television as Masasi Bordeaux Garnet™ (photo below, graph point #1 above) would be classified by this researcher as a light-colored brownish orangey pink Malaya (trade name Imperial Malaya). Malayas of similar color and composition to this Garnet can also found in Sri Lanka and Madagascar.
Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Bright Orange Malaya
Brownish Orange Malaya
Three Malayas with Primary Spessartine Content
Near Colorless Orange Malaya .22ct
Malayas are generally classified by GIA as belonging to the Pyrope-Spessartine species, but our study shows that the majority contain significant amounts of all 3 end-members (Pyrope, Almandine and Spessartine), while some are Pyrope-Almandines and others are Pyrope-Spessartines. Studies by other researchers have also shown that a small amount of Grossular, under 5%, is also commonly present.
On average, Malayas contain more Spessartine than Standard Pyrope, however only 3 samples out of 92 Malayas tested in this study have been found to have Spessartine as the primary component. Two of these Malaya Spessartines were barely over the Pyrope-Spessartine border. Below is the composition graph for 87 Malaya Garnets tested. Point #2 represents the Malaya with the highest Spessartine content.
Near-colorless Malayas are very rare. This researcher has encountered only one example, and such Malayas have not been described anywhere outside this website. The near-colorless gem shown below has a slight orange tint due to manganese. The small size and depth of the gem also contributes to the lighter tone. This gem contains the least amount of Spessartine of any orange Malaya tested (62% Pyrope, 36% Spessartine, 2% Almandine). It has the same major composition as pale pink Malaya, but apparently lacks enough chromium to induce pink as the predominant color. Though almost colorless, this small light-weight gem contains enough manganese to be easily picked up by an N52 magnet.
Malayas with orange color due to Spessartine content are far less common in the marketplace than reddish stones. Orange Malayas can be orangey red, pure orange, brownish orange, pale orange, and near-colorless with just a hint of orange. Some orangey red Malayas will shift color to brownish orange when viewed under fluorescent light.
The 3 strongly colored orange gems pictured below are all from Tanzania. These contain a higher percentage of Spessartine relative to most other Malayas, but each has Pyrope as the primary component. The orangey red Malaya on the left contains nearly as much Spessartine as it does Pyrope (42% Pyrope, 39% Spessartine, 19% Almandine).
Magnetic testing (RIMS analysis) of Masasi gem #1 purchased from JTV reveals a composition of Pyrope and Spessartine in roughly equal proportions: 50% Pyrope, 49% Spessartine, and 1% Almandine. Some Grossular is also likely present. Two other Masasi gems tested were also Pyrope-Spessartines containing 1) 58% Pyrope, 41% Spessartine and 2) 62% Pyrope, 35% Spessartine respectively. Each also contained a small percentage of Almandine. These compositions are similar to that of pale "Imperial" Malayas and pale pink Malayas.
“Imperial” is a trade name or marketing term that is most often applied today to Malaya Garnets that have a brown-orange-pink color (reminiscent of Imperial Topaz) and an intermediate tone that is neither overly dark nor overly pale. Generally, the plot points for Imperial Malayas are centrally located on the Pyralspite ternary, indicating a mixture of Pyrope, Almandine and Spessartine, with Pyrope as the primary component. The composition of Imperials can be loosely differentiated from Red Malayas, which tend to fall near the Pyrope-Almandine line (high Almandine), from pale pink Malayas, which tend to fall near the Pyrope-Spessartine line (high Spessartine), and from orange Malayas, which tend to fall in the right side of the Malaya range (highest Spessartine).
"Umbalite" Garnet (pictured below) is a red Malaya Garnet from Umba Valley, Tanzania. Umbalite is a trade name rather than a distinct variety of Garnet. These Garnets often have an undertone of orange.
Masasi Bordeaux Garnet™
Cause 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 and red of Almandine can be attributed to their primary species content, but the red of Pyrope Garnet is not due to Pyrope (magnesium) content. The red is derived from Almandine (iron) and Uvarovite/Knorringite (chromium). 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 (magnesium), but rather from Spessartine (manganese) in combination with minor Goldmanite (vanadium) and Uvarovite (chromium) content (Schmetzer et. al. 2009).
which impart the green color found in these Grossulars, are derived from slight mixing with Uvarovite (Cr3+), Knorringite (Cr3+) and Goldmanite (V3+) Garnet species. That is to say, chromium and vanadium ions partially replace calcium in the A site of the chemical formula for Grossular. These metals that give green Grossulars all of their color may make up as little as 0.1% of a gem's weight.