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Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral
Tsavorite & Merelani Garnets
Distinguishing 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: Rhodolite is more 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. 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 (with very little red) in daylight. However, color differences may not always be plainly seen, and visual distinctions can at times be difficult and subjective.
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 them from Standard Pyropes, which tend to be pure red and darker in tone.
from Brazil (2.63ct)
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 under reflected light. The same is true of some Pastel Pyropes and Malaya Garnets, as we have already seen in the section on Pastels on page 2.
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
As we have seen, color alone is not a reliable parameter for identifying Garnets. Due to the strong influence of trace chemistry, color does not necessarily predict primary composition. Conversely, primary composition often does not accurately predict color. 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 the 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 minor or trace chemistry (chromium, vanadium and titanium) and/or inter-valence charge transfer processes that are not detectable with a magnetic susceptibility balance. We'll refer to minor chemistry as less than 5% of total composition, and trace chemistry as less than 1%.
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. It is called color shift rather than color change since pink is considered a shade of red. Color Change Pyropes exhibit an actual 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 at the bottom of the Resources and Links page. To review the Garnet classification system developed during this study, go to Gem Garnet Classification
Distinguishing Between Orange Garnets
Spessartine & Hessonite: Among gem Garnets, orange color (due to manganese) is primarily associated with Spessartine and Hessonite. An N-52 magnet quickly distinguishes between them because Hessonite fails to pick up or drag (due to low levels of iron). Hessonite also has a lower refractive index.
Orange Malaya (RI 1.758-1.779)
Malaya & Color Change Garnet: More challenging is distinguishing orange Malayas from orange Color Change Garnets.
Orange Color Change Garnet
Pale Orange Malaya
Pale 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 iron (Almandine), which can be detected with a spectroscope. Orange Color Change Garnet points hug the upper section of the line that connects Pyrope and Spessartine, indicating primary Spessartine (manganese) content and only a small amount of Almandine (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 above graph. The 2 gems have nearly identical refractive indices within the readable 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 N-52 magnet, while Hessonite has a much lower magnetic susceptibility and rarely shows a 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
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 facetable Chrome Pyrope from Standard Pyrope (although some 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 another good separation parameter. Most Chrome Pyropes and Pastel Pyropes show only a Drag response or a weak Pick up response to an N-52 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, as Standard Pyrope tends to show more distinct iron absorption in the green region of the spectrum. A Chelesa filter can also be useful. Chrome Pyrope tends to appear solid red under the filter, while Standard Pyrope has a greenish cast. Magnetic susceptibility measurements are also good indicators, 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 iron absorption. However, pale gems may show no visible absorption spectrum, and 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 pale orange Malayas could just as well be classified as orange Color Change Garnets.
Malaya & Spessartine: Hessonite and Spessartine are not the only orange Garnets. Malaya, Color Change Garnet, and Pastel Pyrope can also be orange. 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.
Reddish Purple Rhodolite
Malaya Garnet & Rhodolite: These 2 varieties overlap in composition. At times they can also 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 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 have distinctive Malaya composition.
This exemplifies 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 or a purple Almandine or a 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 without any distinctions. Color Change Garnets range from 30% 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 Pyrope-Almandines because they are intermediate in the 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 and can be indistinguishable with the naked eye. All pick up with a handheld N-52 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. In addition, typical red Pyrope has a unique broad-banded absorption spectrum that differs from that 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 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 under different light sources. Any hint of purple body color points toward Almandine. Any orange tint points toward Spessartine, and some Spessartine is 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.:
Refractive index is not a reliable indicator for separating orange Malayas from orange Color Change Garnets, as their ranges overlap. Dark to medium orange Malayas (pictured above) show no visible color shift from orange to red, but pale orange Malayas (pictured below) can show color alteration from orange to pink, which could be described as true color change. This is a bit of a quandary, as Malayas are not supposed to exhibit color change, only color shift.
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 Pyrope 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 is an unusually dark purple 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. 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.
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 and Almandine; Rhodolite and Almandine; Color Change Pyrope and Color Change Spessartine. Gems with intermediate compositions that are near the boundary between the 2 species 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 any gem Ugrandite other than Uvarovite can be classified simply as a Grandite, falling along the line that connects Grossular and Andradite. There is also a clear separation between the 2 species, 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 both), the distinctions we adopt are quite arbitrary. There may be little difference in color or composition between a high Pyrope 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 a small variation in trace chemistry and/or inter-valence charge transfer processes that results in a slightly altered appearance.
But gemologists have to draw lines somewhere to separate species and varieties if we are to distinguish and categorize the innumerable variations in Garnets seen within a solid solution series. The challenge is to arrive at a consensus as to which parameters are the most appropriate for separating and naming Garnets.