Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Garnets are some of the most magnetic gemstones, and magnetic testing gives us rare insights into the world of Garnets, revealing how the different species and varieties of Garnet are related. For most people, the term Garnet brings to mind a dark red gem, but Garnets are in fact a diverse group of gemstones, with more species and color varieties than any other type of transparent gemstone.
Two Sub-groups of Garnets
The horizontal axis of the above graph shows Magnetic Susceptibility increasing from zero all the way to 47.5 X 10 (-4) SI, the highest level of magnetism found in gem Garnets. The further a graph point is to the right, the more magnetic the Garnet. Because Garnets are on the order of 100 times more magnetic than allochromatic transparent gemstones, the notation for Garnet has been changed from 10(-6), which is used on the Magnetic Susceptibility Index for Gemstones, to 10(-4). To convert, just move the decimal point over 2 places to the right. For example, 12.5 SI for Rhodolite on the Garnet graph above would read as 1250 SI on our Index.
The vertical axis shows refractive index increasing from RI 1.70 all the way to RI 1.89, which is near the upper limit of refractive index for gem Garnet. The higher the graph point, the higher the RI of the Garnet.
The graph shows two ternaries (triangles), the upper one outlined in green and the lower in red. The green ternary represents Ugrandites, and the red ternary represents Pyralspites. Gemologists separate the 6 gem Garnet species of the Garnet group into these two sub-groups or series. The 3 species in each ternary intermix with each other in what is termed "solid solution series".
Ugrandite is an abbreviation for the Uvarovite-Grossular-Andradite solid solution series. The most familiar Ugrandites are green due to the presence of chromium and/or vanadium. A 2-letter abbreviation for each species name is located at each of the 3 apexes of the ternary, representing the pure state or end-member state of the species. The end-member abbreviation for Grossular is Gr., Andradite is An. and Uvarovite is Uv. Most Ugrandite gems are in solid solution between Grossular and Andradite.
Pyralspite is an abbreviation for the Pyrope-Almandine-Spessartine series. The most common Pyralspites are red primarily due to the presence of ferrous iron (Fe2+), but chromium can also contribute to the red color. The red Pyralspite ternary shows the abbreviations of the 3 end-member species names at the 3 apexes as Al. (Almandine), Sp.(Spessartine) and Py.(Pyrope).
RIMS Graph of Garnet Compositions
Drafted by this author based on the Hoover Diagram
The Hoover RIMS (Refractive Index, Magnetic Susceptibility) diagram below shows typical compositions for Garnets of gem quality. It was drafted by Kirk Feral to serve as a teaching aid for students. The graph was constructed using a free graphing software program called Graph. Red points on the graph represent approximate averages for common Garnet species & varieties based on actual measurements of gem Garnets taken by this researcher. The graph provides a unique visual representation of how various types of Garnets are related. We’ll examine this "Garnet map" section by section in order to gain a better understanding of gem Garnets.
Specific Gravity and Refractive Index
Pink stripes across each ternary represent specific gravity in increments of 0.1. The specific gravity of any Garnet gem can be estimated from the location of its plot point relative to those pink cross bars. For example, the specific gravity of the Almandine point shown on the graph above can be estimated at 4.07.
Conversely, we can use the graph to estimate the refractive index of a Garnet based on its accurate specific gravity measurement. This can be useful for OTL gems like Andradites whose RI's cannot be measured with a standard refractometer.
Specific gravity repesents the density of a gem. The denser the Garnet, the higher the concentration of metallic coloring agents within the gem’s structure, and the higher the magnetic susceptibility.
Refractive index also increases as density increases. This is true not only of Garnets, but of all gemstone species. You can see on the Graph of Garnet Compositions above that Garnets with high RI’s, such as Demantoid and Spessartine, are more magnetic than low RI Garnets, such as Grossular and Pyrope.
The degree of magnetism increases with the refractive index, and this relationship is directly proportional within each of the two series. The graph point on the lower end of the green ternary below represents a green Mali Grossular Garnet oval (pictured below left) with a refractive index of 1.76 and a magnetic susceptibility of 5. The upper point on the same ternary represents a brown Mali oval (pictured below right) with a higher RI 1.785. The brown Mali is twice as magnetic as the green Mali, with a susceptibility of 10.
Magnetism Increases with Refractive Index
There are 15 species of Garnet recognized by mineralogists, but most are of little interest to gemologists. Only 6 Garnet species produce gem-quality crystals that are fashioned into gemstones. These are Almandine, Pyrope, Spessartine, Grossular, Andradite and Uvarovite. Almandine, a red to purplish red gem, is the most common species. Gem-quality Uvarovite is only found as clusters of tiny green crystals (druse), and Uvarovite jewelry is fashioned from druse slabs.
Grossular Garnet Cubic Crystal from Canada
Blue Color Change Garnet
The Hoover diagram is a modification of the Winchell diagram for Garnets, developed by Dr. Horace Winchell in the 1950's as a map for Garnet compositions. Later in 1985, Manson & Stockton published a classic study of Garnets using refractive index, color and spectroscope readings to infer the chemical compositions of individual Garnets. They used an updated Winchell diagram to plot their results.
Dr. D.B. Hoover, along with Bear WIlliams and colleagues, published a new method for identifying Garnets in 2008 using a further modification of the Winchell diagram that involves magnetism as a variable. Hoover's method permits a much more accurate determination of Garnet chemistry than the Manson/Stockton method because it relies on refractive index and magnetic susceptibility measurements, which are both precise quantitative measurements. In the highly regarded Garnet study by Manson/Stockton, two of the parameters used to approximate Garnet compositions were qualitative assessments of color and absorption spectra.
A tiny 1/8" diameter (3mm) neodymium magnet is applied to gems to measure magnetic susceptibility via a Hoover balance.
In the Garnet Composition graph above, parentheses ( ) are around abbreviations for the ions that fill sites A and B, with bold italics highlighting the ion acting as the chromophore. For Almandine, the 2 ions are iron and aluminum, with ferrous iron (Fe2+) as the chromophore. Spessartine has the chromophore manganese (Mn2+), Uvarovite has chromium (Cr3+) and Andradite is colored by ferric iron (Fe3+). Above the parentheses is the predominant color commonly associated with the species, but the colors of 100% pure species are not really known, as Garnet species are never 100% pure. Note that the Pyrope and Grossular species are allochromatic and have no metallic chromophores represented in their chemical compositions.
Ferrous iron (Fe2+) that colors Almandine results in higher magnetic susceptibility for the Almandine pure end-member than does ferric iron (Fe3+) that colors the Andradite pure end-member. We can see on the graph above that the magnetic susceptibility of pure Almandine is calculated at 41, while pure Andradite is only 31. But gems of these Garnets do not approach the pure end-member state, and we find that the actual ranges of magnetic susceptibility for Almandine and Andradite gems are very similar to each other. Spessartine Garnet colored by manganese (Mn2+) has the highest magnetic susceptibility value (47.5) of all the gem Garnet species. The theorectical pure Pyrope and Grossular end-members are completely colorless and have magnetic susceptibility values of 0.
Each of the two Garnet ternaries has a central point from which 3 equal sections (trisections) are drawn, one for each of the 3 Garnet species. When a graph point falls within a particular trisection, we conclude that the gem belongs to that particular Garnet species. As you can see below, the Almandine graph point falls within the Almandine trisection. This particular Almandine graph point represents an average of Almandine Garnet readings that we have so far measured. Actual graph points for individual Almandine gems can theoretically fall anywhere within the Almandine trisection.
Light green Grossular Garnet is in the Ugrandite series.
Light pink Malaya Garnet is in the Pyralspite series.
In addition to red, Garnets are found in every color, from colorless to black, pink, purple, blue, green, orange, yellow, and even iridescent. Some Garnets change color from daylight to incandescent light, much like Alexandrite. There are also hundreds of more subtle color variations among gem-grade Garnets. Some varieties have only been discovered in the last few decades, and some are among the world's rarest gemstones. We'll take a tour of all of them, and present a considerable amount of new information about Garnets never before published.
© 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.
A Map of Garnet Compositions
A Better Understanding of Garnets
What are Garnets?
The Garnet group falls within the family of Orthosilicate minerals, all with related chemical compositions. Other Orthosilicates include Peridot, Zircon, Topaz, Andalusite, and many secondary gems such as Kyanite and Sphene. What all these gemstones share in common is the chemical component SiO4 (one silicon atom per 4 oxygen atoms) in their chemical formulas. The generic formula for Garnet is:
Garnets belong to the cubic crystal system, and therefore gems are isotropic (singly refractive). However, under a polariscope, anomalous double refraction is common in red and green Garnets due to distortion of the isotropic cubic crystal structure during metamorphic conditions of high pressure and high temperature. Garnets are found all over the world. Some of the most unusual gem varieties come from African countries such as Namibia, Tanzania and Madagascar. Garnets are one of the few primary gems that are generally not treated, although Demantoid, and occasionally Hessonite (Bear Wiliams, pers. comm., 2011), are known to be heated to enhance color. The synthetic Garnet GGG is no longer manufactured for gem purposes, and synthetic Garnet YAG is found only in limited quantities.
Understanding Garnets through Magnetism
Garnets are the only common transparent gemstones that show a Pick Up response to an N-52 magnet. They more magnetic than other transparent gems because they generally contain higher concentrations of paramagnetic iron (up to 35% iron oxide by wt.) and/or manganese (up to 40% manganese oxide by wt.). In the case of Uvarovite, chromium (up to 27% chromium oxide by wt.) is the cause of strong magnetism. High magnetic susceptiblity is a key reason we can study Garnets in more detail than is possible with other gemstones. Garnet chemistry can be inferred primarily by determining relative concentrations of iron and manganese.
Identifying Garnets by their chemical composition is often a destructive, complicated and expensive procedure. Consequently, the chemical composition of a particular gem is not known by the average gemologist or student who wants to identify a Garnet by species and variety. Usually refractive index, color and absorption spectra are the primary clues to Garnet identification. This is not the case when you have a Hoover magnetic susceptibility balance at your disposal (described on page 4 of Overview of Magnetism).
With a Hoover balance, in combination with a refractometer and Hoover diagram, an individual Garnet gem can be identified by species and variety, and the chemical composition of the gem can also be accurately estimated in terms of the percentages of the gem's 2 or 3 major species components. This method of analysis has revealed to us that Garnet gems sold by dealers are frequently mis-identified as to species and variety.
The extreme diversity seen in Garnets is the result of different species mixing together in endless variation. We'll see how magnetic testing can reveal exactly how different Garnet species have blended within an individual gem. Based on analysis of over 500 Garnets, we present a new proposed Gem Garnet Classification system, as well as the first Graph of All Gem Garnets showing composition ranges for all known gem Garnets.
Contents of this Section
P.1) A Better Understanding of Garnets
What Are Garnets?
Understanding Garnets through Magnetism
A Map of Garnet Compositions
Two Sub-groups of Garnets
Specific Gravity and Refractive Index
P.2) Why So Many Color Varieties?
Pure End Member Species
Color Change Garnet
P.3) Idiochromatic or Allochromatic?
Cause of Color
Graph of All Gem Garnets
Determining Percentages of 3 End Members
P.4) Rhodolite Garnet
Pastel Pyrope: A New Garnet Variety
Four or More End Members
P.6) Distinguishing Garnet Species and Varieties
Distinguishing Between Orange Garnets
Rhodolite Garnet from Burma
Hessonite Garnet from Nigeria
The bar graph below compares the magnetic susceptibility (degree of magnetic attraction) of most gem Garnet species and varieties in relation to each other. Garnets are presented from least magnetic to most magnetic. The height of each colored bar represents the median measured magnetic susceptibility for the species or color variety.
Relative Magnetic Susceptibilities of Garnets