© Kirk Feral 2012, 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.
Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Tsavorite & Merelani Garnets
Distinguishing Between Garnet Species and Varieties
Ugrandite Garnet gems are easily distinguished from one another. The 3 species and multiple varieties generally do not simultaneously overlap in color, refractive index and magnetic susceptibility. For example, Green Grossular has a different color than orange Hessonite; Hessonite doesn’t share the same colors or brilliance with Mali Garnet; and Mali Garnet doesn’t have the high refractive index and magnetic susceptibility of Demantoid or brown Andradite. Perhaps the most difficult distinction we might have to make among Ugrandites is deciding whether to classify a Green Grossular Garnet that has only a moderate green tone as a Tsavorite or as a Merelani.
Rhodolite: Although Rhodolite is known for its purple color, distinguishing it from other Pyropes with purple color (purple Pastels, purple Malayas and purple Color Change Garnets) can be problematic. The primary composition range of the Rhodolite variety overlaps the composition ranges of four other Pyrope varieties: Standard Pyrope, Malaya Garnet, Pastel Pyrope and Color Change Pyrope. Without a comprehensive chemical analysis to reveal the minor composition of individual gems, we must at times rely on visual appearance (hue, tone, color change or color shift) to distinguish gems of some Pyrope varieties from Rhodolite.
For example, some Rhodolites can occasionally fall within the composition range of Pastel Pyropes, with refractive indices under 1.745 and susceptibilities under 12.5 SI. In these rare instances, we can distinguish the Rhodolite gems by their reddish purple color. Purple Pastels are similar in appearance, but they tend to show a purer purple (pinkish purple rather than reddish purple) in daylight.
Red Malaya & Standard Pyrope: Similar to the example above, some red Malaya Garnets and Standard Pyrope Garnets have overlapping refractive indices and magnetic susceptibilities. The major compositions can be the same for these gems. That means there may be no difference in the proportions of Pyrope, Almandine and Spessartine that make up Standard Pyropes and some red Malayas. In these cases, the lighter tones and off-red shades of red Malayas are the only parameters we have to distinguish this variety from Standard Pyrope, which tends toward darker tones and purer red colors (as shown below).
High Rhodolite
from Brazil (2.63ct)
Low Almandine
from Tanzania (1.47ct)
Chrome Pyrope & Standard Pyrope / Pastel Pyrope & Malaya Garnet: When it comes to Pyrope Garnet, visual appearance alone is often not sufficient to distinguish between the six different varieties within the Pyrope species. Sometimes gems of two different varieties can appear identical, as is the case with some Chrome Pyropes and Standard Pyropes when viewed in reflected daylight. The same is true of some Pastel Pyropes and Malaya Garnets, as is noted in the section on Pastels on page 4 of Garnets.
Color Change Pyrope (lft) &
Color Change Spessartine (rt)
Color Change Pyrope (lft) &
Color Change Spessartine (rt)
Identical Examples of
Chrome Pyrope & Standard Pyrope
Identical Examples of
Pastel Pyrope & Malaya
The Magnetic Susceptibility Boundary that Separates
Chrome Pyrope from Standard Pyrope is SI 10
Purple Pastel Pyrope
Pastel Pyrope and Rhodolite Compositions Can Overlap
Standard Pyrope & Red Malaya
Standard Pyrope and Malaya Compositions Can Overlap
Conclusion
As we have seen, color alone is not a reliable parameter for identifying Garnets by species or variety. Due to the strong influence of trace chemistry, color does not necessarily predict primary composition. Conversely, primary composition often does not accurately predict color (or the phenomenon of color change). Similarly, absorption spectra as seen through a hand held spectroscope are often not reliable indicators of composition.
All Garnet gems are composed of several end-member species. Hanneman proposes we recognize 7 different hybrid species in addition to the 6 primary end-member species. Hybrid species names such as Pyrope-Almandine and Pyrope-Spessartine (adopted by GIA) can be useful, but we would also need to introduce the triple hybrid species Pyrope-Almandine-Spessartine to adequately classify many Pyralspites that contain significant amounts of all 3 end members in that series. This could end up quite confusing, as any particular Malaya, Rhodolite or Pastel Pyrope gem might be classified as belonging to either the Pyrope-Almandine, Pyrope-Spessartine or Pyrope-Almandine-Spessartine species, depending upon its composition. This researcher suggests we recognize only the 6 primary gem Garnet end-members as species when classifying Garnets.
Graph Showing a High Rhodolite & a Low Almandine
Separated by the 50% Boundary Line
Color Change Pyrope and Color Change Spessartine
are Separated by the 50% Boundary Line at SI 24.5.
Pink Malaya & Color Change Pyrope: There is also no difference in major composition between pink Malayas and Color Change Pyropes. Both are varieties of Pyrope that occupy the same compositional range on the Pyralspite ternary. Distinctions between these gems are based solely on differences in color and color phenomena, which can be due to very small variations in trace chemistry (chromium, vanadium and titanium) and/or inter-valence charge transfer processes that are not detectable with a magnetic susceptibility balance. We refer to trace chemistry as metal oxides that make up less than 0.1% by weight of a gems total chemistry.
Pink Malayas (yellow points) and Color Change Pyropes (red points)
Share the Same Range of Composition
Pink Malayas exhibit color shift from pink in daylight to reddish pink in incandescent light. Color Change Pyropes exhibit color change from a non-red colors to red. We have already shown that Pink Malayas do show color change from pink in daylight to yellow in fluorescent light, but incandescent light is the standard light source used to determine color change in gems.
Pink Malaya (2.19ct)
& Yellow Color Change Pyrope (1.75ct)
This concludes the section on Garnet Magnetism. For a list of reading materials about Garnets, see the Garnet Magnetism references on the Resources and Links page. To review the Garnet classification system developed during this study, go to Gem Garnet Classification.
To go to the Magnetic Susceptibility Index for Gemstones, click on this link: Magnetic Index.
Distinguishing Between Orange Garnets
Spessartine & Hessonite: Among gem Garnets, orange color is primarily associated with Spessartine and Hessonite, but Malaya Garnet, Color Change Garnet, and Pastel Pyrope can also be orange in color. An N52 magnet quickly distinguishes between Spessartine and Hessonite, as Hessonite fails to pick up or drag (due to low levels of iron). Hessonite also has a lower refractive index, and often shows a wavy crystal structure under magnification.
Orange Malaya (RI 1.758-1.779)
Orange Spessartine
RI 1.792-1.814
Malaya & Color Change Garnet: More challenging is distinguishing orange Malayas from orange Color Change Garnets. Refractive index is not a reliable indicator for separating orange Malayas from orange Color Change Garnets, as their RI ranges overlap. Dark to medium orange Malayas such as the 10.24ct. orange Malaya Pyrope we tested from GIA's Gubelin collection (pictured below left) shows no color change from orange to red under incandescent light. The orange Color Change Pyrope below right does show color change.
Orange Malaya Pyrope
GIA Gubelin Collection
Orange Color Change Pyrope
Light Orange Malaya
Light Orange Color Change Garnets
As shown below, the orange Malaya points plot more toward the middle of the Pyralspite ternary, indicating primary Pyrope content with a significant amount of both manganese (Spessartine) and ferrous iron (Almandine), which can be detected with a spectroscope. Orange Color Change Garnet graph points hug the upper half of the segment that connects Pyrope and Spessartine, indicating primary Spessartine (manganese) content and only a small amount of Almandine (ferrous iron).
Graph of Orange Garnets
Color Change Garnet & Spessartine: The composition of orange Color Change Garnet can sometimes be very close to standard orange Spessartine, as we can see by the highest red point #38 next to the Spessartine green point #8 in the upper right portion of the graph above. The 2 gems have nearly identical refractive indices within the measurable range of the refractometer fluid. In this case, orange Color Change Garnet (below left, from Madagascar) can only be distinguished from the pure orange Spessartine (below right, from Namibia) by red tones in the body color, and red flash in incandescent light.
Orange Color Change Garnet #38 and Orange Spessartine #8
Pastel Pyrope & Hessonite: Orange Pastel Pyrope is very rare and is easily distinguished by its low refractive index (under 1.745) relative to other orange Pyralspites (above 1.755). However, the refractive index of Pastel Pyrope falls in the same range as that of Hessonite Garnet, which is an orange Grossular Garnet. These gems can be separated by magnetic response. Pastel Pyrope shows a Drag response to an N52 magnet, while Hessonite has a much lower magnetic susceptibility and rarely shows any direct response. Magnification with a loupe or microscope may also reveal structural differences. Large granular inclusions and/or wavy "treacle" patterns are frequently seen in Hessonite.
Orange Pastel Pyrope
Orange Hessonite
A refractive index under 1.745 for Pastels seems to be a consistent parameter for separating these gems from Malayas. An RI under 1.745 also separates Chrome Pyrope gems from Standard Pyrope (although some non-gem mineral specimens of Chrome Pyrope can have much higher RI's). As magnesium (plus chromium in Chrome Pyrope) substitutes for iron, magnetic susceptibility decreases. Magnetic response is also a good separation parameter. Most Chrome Pyropes and Pastel Pyropes show only a Drag response or a very weak Pick up response to an N52 magnet. Most Standard Pyrope and Malaya Garnets of average size show a clear Pick up response.
A hand-held spectroscope seldom reveals definitive differences between varieties of Pyrope, but it can be helpful in separating Chrome from Standard Pyrope. Gems of Chrome Pyrope tend to show a wide band of chromium absorption that obscures the yellow region of the spectrum, while Standard Pyrope gems show a narrower band of chromium absorption in the yellow region, and greater iron absorption in the green region. Magnetic susceptibility measurements are also good parameters for separating these Pyrope varieties, and we can designate SI 10 as a working boundary that separates Chrome Pyrope from Standard Pyrope. To separate Pastel Pyrope from Malaya Garnet, SI 12.5 is the working boundary.
The Magnetic Susceptibility Boundary that Separates Pastel Pyrope from Malaya is SI 12.5
Large Dark Red Malaya 8.44ct
To separate orange Malayas of any shade from orange Color Change Garnets, a spectroscope comes in handy. Orange Malayas have more iron and usually show a band of iron absorption in the green region of the spectrum, while orange Color Change Garnets are more purely Pyrope-Spessartine in content and do not show significant iron absorption. However, light-colored gems may show no visible absorption specta, and chemical composition data may be the only way to make distinctions.
Orange Color Change Garnets tend to fall along the Pyrope-Spessartine line and have higher magnetic susceptibilities than orange Malayas. Arguably, at times there is little difference between the two, and light orange Malayas could just as well be classified as orange Color Change Garnets.
Malaya & Spessartine: Orange Malaya Garnets (from Tanzania) can look similar to orange Spessartines, but Malaya is easily distinguished from Spessartine by its lower refractive index and magnetic susceptibility.
Orange Hessonite
Orange Spessartine
Reddish Purple Rhodolite
Rhodolite and Malaya Garnet can also overlap in composition, and at times they can be difficult to distinguish visually. The primary distinction is purple color. The color and composition of the 2 red gems shown below are very similar (see the graph below right). The subtle purple hue seen in the gem on the left may be the only parameter we can use to characterize it as Rhodolite rather than red Malaya. This is a subjective distinction, as there is no established criteria for the amount of purple that defines Rhodolite as a distinct variety.
Rhodolite and Red Malaya
These 2 Gems Have Similar Composition
Red Malayas that are unusually large in size can be overly dark. These darker Malaya gems can sometimes be visually indistinguishable from Standard Pyrope, yet these can have distinctive Malaya composition, with a significant amount of Spessartine content in addition to Pyrope and Almandine.
This example illustrates how color is not a reliable indicator of species or variety within the Pyralspites. Note: Be aware that in the marketplace any Garnet with purplish red color can be referred to as a Rhodolite whether is a true Rhodolite (Pyrope) or a purple Almandine or a purple Pastel Pyrope.
Color Change Garnet: Now let's look at Color Change Pyrope and Color Change Spessartine. These represent two different species that can have widely divergent compositions, yet these gems are generally referred to collectively as Color Change Garnet with no distinction between species, or they are represented as belonging to a hybrid Pyrope-Spessartine species. Color Change Garnets range from 30% Spessartine all the way to 89% Spessartine, with RI's from 1.742 to 1.792. Most Color Change Garnets with RI's above 1.76 are primarily Spessartine, but refractive index readings and color are not always sufficient to separate the two species. Unfortunately, a spectroscope is not very helpful with pale Garnets like these. But we can distinguish between the 2 species by measuring magnetic susceptibility and determining which side of the 50% composition boundary line a particular gem falls. The boundary point is found at .5, the half-way mark (SI 24.5) on the Pyrope-Spessartine line of the Hoover Diagram below.
If we adhered to parameters for Garnet classification adopted by GIA, we would refer to both purple gems in the above graph as belonging to the Pyrope-Almandines species because they are intermediate in the solid solution series. But as we have shown, there is no need to apply intermediate species names to indicate hybrid composition when we can show on a graph the precise major compositions of individual gems relative to other Garnets in a series.
The two identical gems above are extreme examples. Although Rhodolite and Almandine exist in a continuous series, they represent two different species. As with Standard Pyrope and Almandine, Rhodolite and Almandine can in most instances be separated simply by refractive index.
Red Pyrope (lft), Red Almandine (cntr), Red Spessartine (rt)
Pyrope: Red Pyrope, red Almandine and red Spessartine all derive their primary color from iron (Fe2+) and can be indistinguishable from one another when viewed in reflected light. All pick up with a handheld N52 magnet. Fortunately, Pyrope is recognizable by its lower refractive index. Also, most red Pyrope gems show a noticeably weaker pick-up response than Almandine and Spessartine gems when a magnetic wand is applied (Chrome Pyrope only drags). The magnetic susceptibility range for red Standard Pyrope (SI 1163-1971) is lower than that of Almandine and Spessartine. Under a spectroscope, standard red Pyrope shows broad absorption in the blue region of the spectrum due to chromium in addition to iron, which differs from the absorption spectrum of typical Almandine and Spessartine.
Almandine & Spessartine: These 2 red species are not difficult to distinguish even though their refractive index ranges overlap and their magnetic pick-up forces appear the same. If the gem is red and has a high refractive index that is measurable (under 1.805), we know it is almost certainly Almandine. The RI of Almandine is almost never over the limit of the refractometer fluid. The RI of Red Spessartine is almost always over the the limit (above 1.805). If the limit of your refractometer fluid is less than 1.805, you must rely on other tests to separate these red Garnets.
It is also helpful to inspect gem color through transmitted light (back-lighting). Any hint of purple body color points toward Almandine. Any orange tint points toward Spessartine, and some Spessartines are pure orange. Under a spectroscope, a typical Almandine will show a distinctive 3-band iron absorption in the green-yellow section of the spectrum, while Spessartine absorption appears primarily in the blue-violet section due to manganese. Magnetic susceptibility measurement (SI) is the most definitive test for separating these 2 species. The susceptibility ranges do not overlap, with Spessartine having the highest range of any Garnet species. Almandine SI range is 1926-3094, while Spessartine SI range is 4301-4728.
Light Orange Malaya and Color Change Garnet: The refractive index ranges of these Garnets overlap. Surprisingly, light orange Malayas (pictured below) can show color change from orange to pink. This is a bit of a quandary, as color change is not a phenomenon associated with Malayas.
For example, red gems along the Pyrope-Almandine continuum that have a refractive index of 1.77 or higher can in most cases be classified as Almandine. But some high Standard Pyrope and high Rhodolite gems can also have a refractive index as high as 1.77. How can we tell which species or variety is correct for such intermediate gems? Standard gemological tests are inadequate to make a distinction, and the accepted practice in gemology is to simply classify these as Pyrope-Almandine, a hybrid species name that has been created to solve the dilemma. But we can make precise distinctions by using the RIMS method to plot RI and SI readings on a Hoover diagram. This allows us to definitively separate any 2 species by determining which side of the 50% composition boundary a gem belongs.
Rhodolite & Purple Almandine: As an example, the purplish red gem shown below on the left has an unusually dark purple color for Rhodolite, and it's composition (RI 1.766, SI 18.40) is near the upper limit for the Rhodolite variety. It is identical in color and very similar in composition to the gem on the right (RI 1.772, SI 19.57), which is at the extrapolated lower limit of Almandine. Although there is little meaningful difference between these 2 gems, the slightly lower refractive index and magnetic susceptibility readings for the gem on the left fall just within the Pyrope ternary and distinguish this gem as a high Rhodolite, with Pyrope as the primary species component. The gem on the right is distinguished as a low Almandine, a different species.
Red Almandine (lft) and Red Spessartine (rt)
Pyralspite varieties sometimes exist in an uninterrupted solid solution series that is continuous in chemical composition and color. Examples include: Standard Pyrope to Almandine; Rhodolite to Almandine; Color Change Pyrope to Color Change Spessartine. Gems with intermediate compositions that are near the boundary between the 2 species/varieties can look the same and also have very similar RI's and magnetic susceptibilities.
The 3 Ugrandite species also don’t overlap much in chemical composition. If you review the All Gem Garnet Graph on page 3, you’ll see that all gem Ugrandites other than Uvarovite are simply Grandites, falling along the line that connects Grossular and Andradite. There is a clear separation between Grossular and Andradite, with no true examples of intermediate composition.
Pyralspite Garnet varieties and species are a different matter. They mix and blend with one other much more freely than Ugrandite Garnets. First, let's review how to separate the 3 red Garnet species: Pyrope, Almandine and Spessartine.
Whether the boundaries between Pyralpsite species and varieties are determined by quantitative measurements or visual appearance (or a combination of both), the distinctions we adopt are quite arbitrary. There may be little difference in color or composition between a high Rhodolite and a low Almandine, and the difference between a red Malaya and a Standard Pyrope, or a Rhodolite and a purple Pastel Pyrope, may in some instances be just small a variation in the concentration of chromium/vanadium that results in a slightly altered appearance.
But gemologists must draw lines somewhere if we are to have any useful distinctions between the numerous variations in Garnet chemistry and color that are found among the numerous solid solution series. The challenge is to arrive at a consensus as to which parameters are the most appropriate for separating and naming Garnets.
Pink Malaya
Rhodolite or Almandine?
Answer: Almandine, RI 1.773, SI 21.99
Pastel Pyrope or Malaya?
Answer: Malaya, RI 1.749, SI 18.97
Rhodolite or Almandine?
Answer: Rhodolite, RI 1.748
Pinkish Purple Pastel Pyrope
.5
Rhodolites can also show color shift under incandescent light, but no clear color change as seen in Color Change Garnets that have purple color and refractive indices similar to Rhodolite. However, such color differences may not always be obvious, and visual distinctions between Rhodolite Pyrope, purple Pastel Pyrope and purple Color Change Pyrope can at times be difficult and subjective.