Rhodolite is a variety of Pyrope that was discovered in 1882 in North Carolina, USA. It was named after the local Rhododendron flower, which it resembles in color. Rhodolite Garnet is defined by this purplish color.
Pastel Pyropes have the lowest refractive index range of all gem Garnets except Hydrogrossular Garnet. This is possibly due in part to mixing with Hydrogrossular, something Pastels share in common with other Garnets that have Pyrope-Spessartine content. Pastel graph points occupy a unique section of the Pyralspite ternary near the pure end-member, indicating that the chemical composition of Pastels is distinct from other Garnet varieties. This study suggests that Pastel Pyrope merits classification as a separate variety of Pyrope.
Brownish Red Pastel Pyropes from Africa
Light Pink Pastel Pyrope, (Tanzania) 3.34ct
Daylight & Fluorescent Light
Pastel Pyrope Graph Points (RI 1.723- 1.745, SI 6.18- 12.36)
Four or More End Members
The purple color of some Pastels (as illustrated by the 2 oval gems shown below) is lighter in tone, with a purer purple color than the reddish purple of Rhodolite. The pink color component is due to iron and chromium/vanadium, while the blue component that combines with pink to create the purple color is linked to higher Spessartine (Mn2+), lower Almandine (Fe2+), and possibly to lower titanium content than in Rhodolite. Purple Pastels like those pictured below exhibit color change from purple in daylight to pink in incandescent light. Most Pastels originate in Tanzania, Madagascar and Sri Lanka. Pastel Pyrope also exists in the form of megablasts (nodules of metamorphic crystal) in the Italian Alps. These megablasts are reported to have purple to pink centers and near-colorless rims. The near-colorless outer layer can be up to 98% pure Pyrope.
Purple Pastel Pyrope
For example, Pyrope Garnets of any variety contain primarily the Pyrope end member along with some Almandine and possibly Spessartine, but they can also contain Grossular, Andradite and Uvarovite. The Grossular content in Pyrope gems is mostly under 5%, but as high as 20% for non-gem Pyrope has been reported by researhcers. Below is a hypothetical example of the composition of a Standard Pyrope gem, showing 4 end members, with Grossular content at 5%.
Chrome Pyrope Graph Points
Chrome Pyrope 0.59ct.
Appears "Crimson" Red
in Transmitted Light
We could also draw a Pyrope-Uvarovite-Almandine ternary to estimate the chromium content for Chrome Pyrope. Graph points positioned outside a ternary have outside composition, but graph points inside the boundaries of a ternary can also have outside composition. Any Garnet graph point that falls on or within the Pyralspite ternary might also fall on a Pyrope-Grossular-Spessartine ternary or a Grossular-Almandine-Spessatine ternary. It's an educated guess as to which end members and how many end members might be present in small amounts in a particular Garnet gem.
Rhodolite Graph Points
"Raspberry" Pink Rhodolite
Although every Garnet, whether a Pyralspite or a Ugrandite, is primarily a mixture of two or three end members from its own series, mixing between two series can also occur, resulting in fourth, fifth and sixth components in minor amounts.
© 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.
Pastel Pyrope: A New Garnet Variety
Pastel Pyrope is a rare and often beautiful variety of Pyrope that is virtually unknown to gemologists. Gems are typically lighter in color compared to Standard Pyrope and Chrome Pyrope, with colors ranging from pink to purple to light brownish red (maroon), and even orangey red. We have also encountered a rare Pastel Pyrope that is primarily yellow in daylight. No blue or green Pastels have yet been encountered. Pastels differ from Standard Pyropes and Malaya Garnets by their higher Pyrope content (i.e. lower refractive index & lower magnetic susceptibility). Unlike most red Garnets, Pastels are not picked up by an N-52 magnet. Instead, they show only a Drag response.
Pastels are difficult to find not only because they are rare, but because they are never sold by that name. All Pastels collected for this study were sold as Rhodolite, Malaya, Umbalite, Color Change or Grossular Garnets. A yellow Pastel Pyrope was sold as Imperial Hessonite. The Pastel Pyropes were subsequently distinguished from other Garnet varieties by their Drag responses (rather than Pick Up responses) to an N-52 magnet, and by their compositions as determined by the RIMS method. In the photo below, purple gems in the top row were purchased as Rhodolites, middle row pink gems were purchased as pink Malayas, and bottom row reddish gems were purchased as "Imperial" Malayas.
Light pink Pastel gems like the ones shown above and below are rare - much rarer than pale pink Malaya Garnets. As with pink Malaya Garnets, most pink Pastel Pyropes derive their colors primarily from chromium. As with Chrome Pyrope, unique geologic conditions of formation among Pastels result in unusually low iron and manganese content. Light pink Pastels from Tanzania such as the gem pictured below exhibit color change from pink in daylight to yellow in fluorescent light, as do light pink Malaya Garnets. The yellow color component is due to manganese. Light pink Pastels can look identical to light pink Malayas, and maroon Pastels can look identical to "Imperial" Malayas, but refractive indices and and magnetic susceptibilities for Pastels are lower than for Malayas (i.e. Pastels have higher magnesium content).
Any variety of Pyrope Garnet (Standard, Chrome, Malaya, Rhodolite, Color Change and Pastel) can contain some Grossular, where calcium substitutes for magnesium in the A site of the Pyrope chemical formula. Similarly, Almandine Garnet commonly mixes to a small degree with Andradite Garnet, which is in the Ugrandite series above it. Therefore Almandine graph points are sometimes positioned above the Pyrope-Almandine boundary line (as seen in the All Gem Garnet graph on the previous page).
Because there is some blending between Pyralspites and Ugrandites, especially between the Pyrope and Grossular species, additional ternaries can be drawn to reflect this. To calculate Grossular content in Chrome Pyrope, we would need to draw a Pyrope-Grossular-Almandine ternary as shown below in red.
Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Orangey Red Pastel Pyrope
Although color and composition of Pastels can be continuous with that of other Pyrope varieties (Malaya, Rhodolite and Color Change), Pastels are among the purest Pyropes, with high magnesium content, low iron content and low manganese content. The unique identification parameters established here for this variety are:
1) Color saturation is usually lighter than other Pyropes
2) Low refractive index range: RI 1.726 - 1.745
3) Drag response due to a low magnetic susceptibility range: SI 6.20- 12.35 X 10(-4)
Pastels in this study have compositions ranging from 74% to 87% Pyrope. Most gems of average size show a Drag response, which separates them from all other Pyralspite Garnets except Chrome Pyrope. Higher refractive index, higher SI values and Pick-up responses in light-colored Pyropes indicate Malaya Garnet, Rhodolite or Color Change Garnet rather than Pastel Pyrope (although some Rhodolites and Malayas barely pick up).
Pastel Pyrope Garnets
Red Pastel Pyrope 15.63 ct
As with other Pyrope varieties, the Pick-up response can be relatively weak, and Rhodolites at the extreme low end of the range may only show a Drag response. The graph below shows Rhodolite graph points. Like Malaya Garnet, Rhodolite is often a mixture of Pyrope, Almandine and Spessartine. Most points fall well inside the boundaries of the Pyralpsite ternary. Point positioning also tells us that this variety of Pyrope Garnet is not just distinguished by its unique purple color (which may be influenced by trace chemistry), but also by its major composition, which tends to involve more Spessartine (manganese) and less Almandine (iron) than what is seen in Standard Pyrope. The range of composition also indicates that most Rhodolites are closer to the pure Pyrope end member than are Standard Pyropes, meaning Rhodolites have higher magnesium content than Standard Pyropes.
Rhodolite color is purplish red to reddish purple or pinkish purple. Color intensity can vary from light (below left) to dark (below right). Purple color is the result of the ratio of chromium/vanadium (pink component) to iron and manganese (blue component), with iron content lower than in Standard Pyrope.
Gems that are reddish purple (2 photos above) with a predominant hue of purple tend to show color shift to pink or red when viewed under incandescent light. Rhodolites that are purplish red (2 photos below) have a predominant hue of red or pink in daylight. Gem dealers use "fruity" names to refer to these Rhodolites, such as "Cranberry" Rhodolite, "Raspberry" Rhodlite, and "Cherry" Rhodolite.
Light Purple Rhodolite
Dark Purple Rhodolite
"Cranberry" Red Rhodolite
(Sri Lanka 3.83ct)
The Uvarovite/Knorringite content, plus some Grossular content, represent multiple component species within the Chrome Pyrope mix. These additional end members place Chrome Pyrope graph points above the Pyralspite ternary toward the Ugrandite ternary so that the points are positioned between the two, as seen below. The points move upward as higher chromium content raises the gem density and refractive index. The upward positioning of Chrome Pyrope points could also be in part be due to the lower magnetic susceptibility of chromium as it replaces iron, moving the points to the left and away from the Pyrope-Almandine line.
Chrome Pyropes, which include Anthill Garnets, have even lower magnetic susceptibilities than Pastel Pyropes, and these gems also show a Drag response rather than a Pick-up response (although very small gems pick up). The refractive index range is very similar to that of Pastel Pyrope. Chrome Pyropes typically have a deeper red color than Standard Pyrope or red Pastel Pyrope due to high chromium content derived from Uvarovite/Knorringite. The coloring agents chromium and iron are the same ones that give Ruby its blood-red color. Chromium content has been reported to be in the range of 4% - 8% chromium oxide for most gem-grade Chrome Pyrope. This is considerably more chromium than what is found in other Garnets, and is likely responsible for a small percentage of the total magnetic susceptibility.
3 Chrome Pyropes (center gem is 0.97ct)
Purple Pastel Pyrope
(Sri Lanka) 2.1ct
If we categorize Pastel Pyropes according to their unique composition rather than the light color saturation that their name suggests, then we find that not all Pastel Pyropes are pastel in color. Some can appear dark red (as pictured below), similar in color to red Malayas or Standard Pyropes. Red gems with Pastel Pyrope composition derive color mainly from Fe2+, and contain very little Spessartine. Their graph points hug the Pyrope-Almandine join at the low end of the Pyralspite ternary, and these gems have major compositions similar to that of Chrome Pyropes (which are discussed below), but with a much lower concentration of chromium than Chrome Pyropes. Like Pastel Pyropes, Chrome Pyropes show a Drag response rather than a Pick-up response.
Red Pastel Pyrope 18.09 ct
Aside from our study presented here, nothing has been written about Pastels since Carol Stockton first published a short paper titled Pastel Pyropes in the Summer 1988 edition of Gems & Gemology. So far, we have collected and graphed 38 Pastels in an attempt to learn more about the composition of these elusive gems.
Pink Pastel Pyrope, Nearly 87% Pyrope
Light Pink Pastel Pyrope, 78% Pyrope
The Pastel Pyrope with the highest Pyrope content tested in our study is shown below (left). This small orangey pink gem from Bekily, Madagascar has a low refractive index of only 1.728, and is nearly 87% Pyrope. Typical red Standard Pyrope averages 68% Pyrope. This Pastel gem is colored primarily by iron and manganese. It shows no UV fluorescence due to chromium. The 1.31ct. light pink Pastel Pyrope trillion from Tanzania shown below (right) is one of the palest Pastel Pyropes we have encountered. It's light pink color is primarily due to chromium (red UV fluorescence), and this gem is 78% Pyrope. Although lighter in color than the oval on the left, the trillion is more magnetic due to higher manganese content (Mn2+, Spessartine). The manganese contributes little if anything to color. Rutile needle inclusions are common in Pastel Pyropes.
Calculating accurate percentages of fourth and fifth end members using the RIMS method would require additional y-axis variables such as specific gravity or unit cell length, and would involve solving simultaneous equations. Such calculations are well beyond the scope and purpose of this website.
Fortunately, a good understanding of Garnet composition does not require any calculations or knowledge of possible fourth or fifth end members. Just looking at the positions of graph points on the All Gem Garnet graph gives us an instant snapshot of how different species and varieties intermix within a solid solution series. We can visually determine which two or three Garnet end members are involved in any individual gem, and accurately estimate their proportions.
Now let's take a tour of the Ugrandite Garnets on the next page.
Chrome Pyrope, 1.86ct.
Some key identification parameters for Chrome Pyrope are:
1) Vivid red, orange-red or purple-red color in transmitted light
2) Chromium absorption spectrum visible with a spectroscope
2) Low refractive index range: RI 1.733- 1.746
3) Drag response due to a low magnetic susceptibility range: SI 4.55 – 10 X 10(-4)
Key identification parameters for Rhodolite are:
1) Reddish purple or purplish red daylight color
2) Refractive index range: RI 1.739- 1.766
3) Pick-up response due to a moderate magnetic susceptibility range: SI 10.6 – 18.9 X 10(-4)
Most light pink Pastel Pyropes, such as the pink gem from Tanzania shown below, fluoresce pink or red under long wave UV light. This is due to the relatively low iron content of Pastel Pyropes, permitting chromium fluorescence to be visible. Other Pyralspite Garnets we have found that fluoresce pink or red under UV light are some light pink Malaya Garnets from Tanzania, as well as some low-iron Color Change Garnets. All these fluorescent Garnets also glow pink under a Chelsea filter due to chromium content. The iron content in Pyralspite Garnets that fluoresce is very low, with most of the magnetic susceptibility coming from manganese, which does not quench chromium fluorescence.
The gem pictured below has relatively low iron content, with a composition of approximately 84% Pyrope, 7% Almandine and 9% Spessartine. Almandine (iron) content may be even lower if Grossular content (up to 5%) is also present. The percentage of chromium in this gem, and in any Pyralspite Garnet, is difficult to assess with a magnetic susceptibility balance or a spectrometer due to the masking effect of iron and manganese.
Pink Pastel Pyrope, (Tanzania), 6.43ct
Daylight & Long Wave UV Light
One highly unusual Pastel Pyrope (pictured below) discovered in our Garnet study showed yellow as the primary daylight color, with pink as a secondary daylight color. This gem was sold as Hessonite Garnet (RI 1.733). It is the only Pastel Pyrope we have encountered that exhibits yellow color in daylight. This pale gem is approximately 76% Pyrope and 24% Spessartine in composition, with very little iron from Almandine. Spectrometer analysis confirms high manganese and low iron content. The mechanism responsible for yellow daylight color in this gem in contrast to pink daylight color in other Pastels that we have analyzed with very similar major composition is not known, but it likely involves a greater proportion of manganese in relation to chromium. The yellow color is derived from manganese, and the pink color is likely derived primarily, or perhaps entirely, from chromium. This gem fluoresces red in long wave UV light due to chromium.
Yellow Pastel Pyrope, 2.03ct
When using the RIMS method, the presence of fourth and fifth end members in significant percentages will diminish to some degree the accuracy of 3-end-member calculations for Garnet composition, as well as the accuracy of graph point positioning. For example, using the RIMS method to calculate the composition of the purple Pastel Pyrope from Tanzania shown below, we determined the gem is 79% Pyrope, 11% Almandine and 10% Spessartine. Electron microprobe analysis of the same material at Caltech found similar end member percentages, but with 5% Grossular present in addition to the 3 Pyralspite end members, as well as 1% Uvarovite/Goldmanite (chromium/vanadium) and an un-assigned remainder of about 2%. These additional components reduce the calculated percentage of the primary component- Pyrope- from 79% to 69%. The electron microprobe precentages were: 69% Pyrope, 12% Almandine, 11% Spessartine, 5% Grossular, 1% Uvarovite/Goldmanite, 2% un-assigned.
Pastel Pyrope with 5% Grossular Content