Magnetism in Gemstones
An Effective Tool for Gem Identification
Ugrandites are referred to as the calcium Garnets. All 3 gem species (Uvarovite, Grossular and Andradite) have calcium in the A site of their chemical formulas. Like Pyralspites, they exhibit a wide diversity of color, particularly among Grossular Garnets. Small amounts of chromium from Uvarovite & Knorringite, and vanadium from Goldmanite, are responsible for the green colors we see throughout the Ugrandite Garnets. Within Ugrandite gems, iron is a weak chromophore. Iron (Fe3+) can create strong magnetism while remaining cryptic (colorless).
Merelani Grossular Garnets
Demantoid: Green Andradites are called Demantoid Garnets. They were first discovered in the Ural Mountains of Russia in the mid-1800’s, and today they are also commercially mined in Madagascar and Namibia. Dispersion of light in these gems is even greater than in Diamond, and transparent green Demantoids are the most valuable Garnets in the world, particularly those with dark green color (Chrome Demantoids) derived primarily from chromium/vanadium. The best Chrome Demantoids can sell for $10,000 per carat. Demantoid colors range from brownish green to yellowish green to mint green to dark green. In the graph above, the lowest graph point represents a dark green Chrome Demantoid.
Demantoids are the only natural transparent green gemstones of any kind that pick up with a magnetic wand (with the exception of rare green Color Change Garnet). The high magnetic susceptibility is due to iron (Fe3+).
Brown and Yellow Andradite: Many Andradites are brown to reddish brown, and on rare occasion, red. Others are yellow, with the trade name Topazolite. Color is likely due entirely to inter-valence charge transfer (iron-titanium and possibly iron-iron, Fe2+-Fe3+). Some brown and yellow Andradites exhibit color change toward red when viewed under incandescent light. All these warm colors of Andradite are grouped as Brown Andradite on the graph above, and all fall along the Grossular-Andradite line.
"Topazolite" from Arizona
Melanite: Black Andradite is called Melanite or Titanium Andradite, and it is generally opaque with a high luster. Melanite forms a solid solution series with Schorlomite, which is a non-gem Garnet that has titanium in the B site of its chemical formula. Titanium, iron and manganese contribute to the black color through inter-valence charge transfer (Fe2+-Ti4+, Mn2+-Ti4+, and Fe2+-Fe3+). No Melanite graph points are shown on the graph above. It is probable that points fall along the line connecting Andradite and Schorlomite, which is for the most part too high to be visible on the graph. There is a wide variance in magnetic susceptibility of black Garnet crystals tested in our study, varying from paramagnetic Melanite crystals to ferromagnetic crystals with extremely high iron content.
Melanite Rough Crystals from Morocco
Iridescent Andradite: One of the rarest Garnets is Iridescent Andradite, also called Rainbow Andradite. It has been mined in Sonora (Mexico), New Mexico (USA), and Nara (Japan). This is an opaque brown Garnet that reflects rainbow colors at the surface when light hits at just the right angle. The rainbow color is due to interference colors and to light diffraction within stratified growth layers (Hainschwang and Notari 2006). This variety of Andradite has the highest magnetic susceptibility and is closer to pure Andradite than any other variety we tested, with a composition of 96% Andradite and 4% Grossular.
Iridescent Andradite Rough from Japan
Chrome Demantoid Garnets
Andradite Graph Points
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Andradite & Uvarovite
Andradite has the highest refractive index of all gem Garnets, and due to its high iron content, it is by far the most magnetic of the 3 Ugrandite species. The refractive indices of Andradite are over the limit of a standard refractometer. So how do we plot composition on a RIMS graph without RI readings? The Andradite graph point positions shown below were approximated based on the measured magnetic susceptibility in inferred proximity to the Grossular-Andradite join. This is based on reports by other researchers that Andradite is in solid solution series with Grossular Garnet rather than with Uvarovite.
Andradite Garnets are found in a range of colors: brown (common Andradite), red, yellow (Topazolite), green (Demantoid), color change, iridescent (Rainbow Garnet), black (Melanite), gray, and in rare instances near- colorless. Colors other than green may involve iron to titanium (Fe2+-Ti4+) and iron to iron (Fe2+-Fe3+) inter-valence charge transfer, while green color is due to iron (Fe3+, yellowish green) and chromium/vanadium (pure green and bluish green).
The concentration of iron (Fe3+) in Andradite can be up to 29% iron oxide (Fe2O3) by weight, high enough to create pick-up responses in all Andradite gems. The concentration of iron is too high to permit any UV fluorescence from chromium, but a Chelsea filter reveals that traces of chromium are common in brown, yellow and green Andradite gems. Although the concentration of iron (Fe3+) is high in all Andradites, iron in some cases creates only light colors, as we find in Topazolite and some light yellow-green Demandtoids. The average composition of Andradite gems that we tested is 81% Andradite and 19% Grossular.
Most Uvarovite Garnet is found in druse form (tiny crystals across the surface of a rock). Uvarovite crystals are generally too small to be faceted as gems, but bright green druse specimens on matrix are often fashioned into jewelry. Uvarovite was discovered in the Ural Mountains of Russia, as was green Demantoid. The majority of druse gems offered on the gem market are from Russia, although Uvarovite is found in other locations such as Quebec, Finland, Afhganistan, South Africa and California.
The Uvarovite species is also referred to as Chromium Garnet. Chromium (up to 27% chromium oxide by weight) rather than iron is the idiochromatic coloring agent, imparting a deep green color and strong magnetism to all examples of this species. Vanadium (from Goldmanite) may also contribute to the green color, and in some cases may be the primary coloring agent. Crystals are mostly opaque, with some transparency at the surface. Chromium ions are rarely magnetically detectable in any gemstone, but because chromium is in such high concentration in this species, we encounter high magnetic susceptibility that can be measured. Most crystals appear bright red under a Chelsea filter, an indication of the high chromium content, but concentration quenching by high levels of chromium prevents any fluorescence under UV light.
Druse specimens attached to matrix are generally too heavy to show a direct response to a magnet. Although not transparent, the small size of druse crystals permits them to transmit more light than larger Uvarovite crystals, and druse specimens appear brighter green in color. Druse crystals are too small to be measured for magnetic susceptibility with a Hoover balance.
Large Uvarovite Crystal in Matrix
For our study, a large 1.78ct (4mm) crystal from Finland (above left) and a large 3mm crystal from Afghanistan (above right) were tested for composition using the RIMS method. The crystal from Afghanistan was embedded in host rock that appears to contain Quartz and Chromite, a mineral rich in chromium and iron.
As with all Garnet species, pure end-member Uvarovite has not been found in nature. Our Uvarovite crystals have a lower measured magnetic susceptibility (SI 9.09 and SI 9.98) than the hypothetical pure end-member (SI 12.75 on the Hoover Diagram). Uvarovite is known to mineralogists to be in solid solution with Grossular and to some extent with Pyrope. In most cases, very little Andradite (i.e. Fe3+ iron) is present.
Therefore we use a Uvarovite-Grossular-Pyrope ternary to plot our graph point rather than the standard Uvarovite-Grossular-Andradite ternary. We extrapolate the over-the-limit refractive index based on proximity to the line connecting Grossular to Uvarovite at the measured level of magnetic susceptibility. If the Finland crystal is colored by vanadium from Goldmanite, then a ternary encompassing Goldmanite would be more appropriate.
The resulting RIMS composition indicates approximately 71% Uvarovite and 29% Grossular for the crystal from Finland, and a higher Uvarovite content of 77% and remaining 23% Grossular for the crystal from Afghanistan. Both also probably contain a small percentage of Pyrope (i.e. magnesium). On the graph below, you’ll find the dark green graph points of our samples situated below the pure end-member along the Uvarovite (Uv.) and Grossular (Gr.) join. Uvarovite can be referred to as Gruvite Garnet, a blend of Grossular and Uvarovite. The dotted line is used indicate the solid solution between Uvarovite and Pyrope. These Uvarovite specimens are the only Gruvite Garnets found in our Garnet study.
Graph Showing Uvarovite Composition
Large Uvarovite Crystal
(4mm, 1.78ct, Finland)
Chemical analyses of Uvarovite samples presented in Deer, Howie and Zussman's 1982 book Orthosilicates indicate that Uvarovite compositions can fall anywhere along the Grossular-Uvarovite join between 50% and 91% Uvarovite. Some Uvarovite specimens were shown to have high Pyrope content, while others had higher Andradite content.
Those researchers (D,H & Z) also indicate that some mineral specimens referred to as Uvarovite are actually high-chromium Grossulars that have Grossular as the primary component. Like Uvarovites, they could be referred to as Gruvites. Such unusual Gruvite Grossulars would have their RIMS graph points within the Grossular tri-section of the Ugrandite ternary along the Grossular-Uvarovite join, with Uvarovite comprising less than 50% of the total composition. No Gruvite Grossular Garnet gems have been encountered in our study of Garnets.
Near-Colorless Andradite: The near-colorless Andradite pictured below left is documented by the RRUFF Project and is considered to be a very pale Demantoid. All the apparent color is likely derived from iron (Fe3+) rather than from chromium/vanadium. Although we have not tested this gem, it qualifies as the only near-colorless gemstone that would be picked up by an N52 magnet.
It might seem unlikely that a gemstone with such high iron content as Andradite could be near-colorless, but this is another example of how iron in the valence state of Fe3+ is by itself a weak chromophore. The iron in the near-colorless gem (left) is visible only as a very pale yellowish green. We have not encountered any Demantoid Garnets with so little color. The Demantoid with the lightest color that we have tested is shown below right. This tiny light green gem was mined in Quebec, Canada, and derives its pale color primarily from iron (Fe3+). It has the lowest Andradite content of all Andradites tested, with a compostiion of 69% Andradite and 31% Grossular.
(Photo by William W. Pinch, Courtesy of rruff.info)
(Crystals under 0.5mm, Russia)
Large individual Uvarovite crystals are quite rare. These larger crystals (over 1mm) are very dark green in color, mostly opaque and very rarely faceted. Such crystals show a Drag response due to high chromium/vanadium content, and single crystals under 1ct in weight can show a Pick-up response. Some very rare crystals over 10 mm in size have been found in Finland, and relatively large crystals are occasionally seen from Russia and more recently from Afghanistan. The crystal from Finland shown below (left) does not glow red under the Chelsea filter, suggesting it is colored primarily by vanadium (from the Goldmanite end member species).
(Crystals up to 1mm, Russia)
All gem Ugrandites (except Uvarovite and possibly Melanite) can be referred to as Grandites, with major mixing in varying proportions of only 2 end member species, Grossular and Andradite.
Light Green Andradite
Demantoids are also the only Garnets that are commonly treated, with low heat being applied to drive out the yellow and brown color. Heat alters the valence state of iron from Fe2+ to Fe3+, and may eliminate intervalence charge transfer processes that produce yellow and brown color in Andradite. The trivalent iron (Fe3+) remaining in green Demantoid after heat treatment is yellowish green. Dark green color in Chrome Demantoids is due to a small amount of chromium/vanadium that is present in addition to the iron, and these Garnets show a red reaction under a Chelsea filter.