Pastel Pyrope

All Garnet gems are mixtures of several Garnet species. The species name of this Garnet indicates the primary component of these gems. The trade name is the common name used by most sellers. The scientific classification of this Garnet by species, variety and color is based on my proposed classification system for gem Garnets, as presented in the Garnet Magnetism section of my educational website www.gemstonemagnetism.com.

The unique identification of this particular Garnet was achieved through laboratory measurements of magnetic susceptibility and refractive index. These two measurements intersect at a point on the Hoover Diagram, and the location of that graph point identifies the primary species, variety and major chemical composition of these Garnets relative to 500 other Garnets that have been tested in my lab. Magnetic susceptibility measurements were taken with a Hoover magnetic susceptibility balance constructed and calibrated by me. The Hoover balance is not commercially available.

Composition Report 11/10/2014

Sample Description:

Three samples: 1) 2.33ct faceted gem, 2) 2.35ct rough with one polished surface, 3) 1.56ct rough with one polished surface

Observations:

Magnification: Fine needle inclusions, likely Rutile, are densely distributed throughout samples 1 and 3, while sample #2 is relatively clean. Needle inclusions are common in Pyrope and Almandine, and dense distribution of such inclusions has been note previously in pale pink Pastel Pyropes from Tanzania.

Polariscope: All 3 samples show ADR (anomalous double refraction). This is common among eclogitic Pyralspites from Tanzania, Sri Lanka and southern Madagascar. The phenomenon is reported as the result of deformation of the crystal lattice under metamorphic conditions (high pressure and temperature).

Color: The samples are pinkish purple in daylight, and show color change from pink in warm incandescent light to purple in cool LED light. Flashes of red can also be seen in daylight along facet edges. Nearly identical daylight colors and color change phenomena have been previously noted in our lab in a number of other purple Pastel Pyropes from Tanzania, Sri Lanka and southern Madagascar.

Testing Methods:

Garnet composition was determined using the RIMS (Refractive Index Magnetic Susceptibility) method, as developed by D.B. Hoover. The RIMS method allows us to infer Garnet composition in terms of the percentages of the 3 major Garnet end-member components. In Pyralspites, the 3 major components are Almandine (iron), Spessartine (manganese) and Pyrope (magnesium). The RIMS method does not allow us to detect components containing chromium and vanadium when these end-member components are present in trace amounts. For a detailed description of the RIMs method, see Hoover, D.B. (2011) "Determining Garnet Composition from Magnetic Susceptibility and Other Properties". Gems & Gemology 47(4) p. 272-285.

Composition Results: Average of the 3 samples is Py79Alm11Sp10 (79% Pyrope, 11% Almandine, 10% Spessartine). See attached graph #1 showing RIMS graph points for the 3 samples.

Sample 1) Refractive Index 1.736, Magnetic Susceptibility 9.81 X 10-4 SI (International System of Units). Calculated composition Py78Alm11Sp12

Sample 2) Refractive Index 1.736, Magnetic Susceptibility 9.85 X 10-4 SI. Calculated composition Py78Alm11Sp12

Sample 3) Refractive Index 1.736, Magnetic Susceptibility 9.37 X 10-4 SI. Calculated composition Py80Alm11Sp9

These Garnets may also contain a small percentage of Grossular (likely under 5%). Any Grossular percentage would correspondingly reduce the relative percentages of the 3 major components.

Discussion:

The Garnet material tested for this report is high-magnesium Pyrope containing roughly equal amounts of Almandine and Spessartine. The variety name is Pastel Pyrope, as first described by Carol Stockton in 1988. Our research using the RIMS method on 30 Pastel Pyrope gems indicates that this variety has a composition that is unique among Pyralspites. Still, Pastel Pyrope is not yet recognized by GIA or any other gemological or mineralogical institution as a Garnet variety. See attached graph #2 showing 30 Pastel Pyrope compositions, and graph #3 showing Pastel Pyrope in relation to all other gem Garnet species and varieties.

Pastels have the lowest refractive indices of all gem-quality Garnet species and varieties other than Hydrogrossular. Quantitative measurements of magnetic susceptibility show that Pastel Pyrope gems are less magnetic than all other Pyralspite varieties except Chrome Pyrope. Pastel gems can often be separated from other Pyalpsites (except Chrome Pyrope) by their Drag response to an N52 neodymium magnet. Gems of any other Pyralspite variety, when of small to moderate size, will be picked up by a neodymium magnet. However, Pyralspite gems and rough stones greater than 4 or 5 carats may be too heavy to show a Pick-up response.

Pastels are found in a variety of colors. They are rare and difficult to find. Purple Pastels such as the samples tested here may be mistaken for light-colored Rhodolite, which can at times also show color change from purple to pink. Rhodolite Garnet, however, is more reddish in daylight and has higher iron content. Depending on color, Pastel Pyrope gems are incorrectly identified and sold as Rhodolite, Malaya Garnet, Color Change Garnet and Grossular Garnet, but never as Pastel Pyrope.