Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral
Natural Blue Sapphire,
Sri Lanka, Weak, (SI < 20)
Iron is involved in the coloration of synthetic blue Sapphire as it is in natural blue Sapphire, so how do we explain differences in magnetic response? Through the process of inter-valence charge transfer, ions of iron (Fe2+) and titanium (Ti4+) interact with each other to cause blue color in Sapphire. Fe2+ loses an electron to become Fe3+, and Ti4+ gains the electron to become Ti3+. This process is such a strong chromophore that only a trace amount of iron (too low to be magnetically detectable) is all that is necessary to create vivid blue color through Fe2+-Ti4+ charge transfer.
In most natural blue Sapphires there is also some "surplus" iron, perhaps mostly Fe3+. This additional iron may exist as cryptic (colorless) iron, or possibly become involved in iron to iron (Fe2+-Fe3+) charge transfer, which may modify blue color. The over-dark blue color that is often seen in blue Sapphires with high iron content is due to maximized iron to titaniun (Fe2+-Ti4+) charge transfer, and also possibly to Fe2+-Fe3+ charge transfer. Additional iron not involved in Fe2+-Ti4+ charge transfer or Fe2+-Fe3+ charge transfer is probably cryptic and likely responisble for all of the detectable magnetic attraction seen in most natural blue Sapphires. This extra iron can also be detected with a spectrometer, as shown in the graph below. In synthetic Sapphires, there is no detectable "excess" iron present, only a small amount of iron in Fe2+-Ti4+ charge transfer, and therefore we see no attraction to a magnet.
Natural vs. Synthetic: Natural Sapphires that are pink, purple or blue can at times be diamagnetic. Therefore gems of these colors cannot reliably be distinguished by magnetic response from synthetic Sapphires. However, natural blue Sapphires most often do show a weak to moderate magnetic response due to iron, and such responses definitively separate them from synthetic blue Sapphires, which are always inert (diamagnetic). We can say that any magnetic attraction rules out synthetic origin for blue Sapphire, but a diamagnetic response does not rule out natural origin.
Vivid Blue Sapphire
S. Madagascar, Weak (SI 22)
Natural vs. Synthetic: One way to distinguish between natural and synthetic transparent Corundum is to look for inclusions. Unlike natural Corundum, synthetic Corundum generally has no inclusions that can be seen with a loupe or gem microscope. A magnetic wand can also provide a useful means of distinction. As we have noted, "fancy color" natural Sapphires with green, yellow, orange, and Padparadscha colors are often weakly to moderately magnetic due to iron ions involved in color. Synthetic Sapphires of all colors lack sufficient iron to show any attraction to a magnet. They are always diamagnetic, and any magnetic attraction indicates natural origin.
Natural Padparadscha Sapphire
Brazil, Weak (SI 43)
Synthetic Padparadscha Sapphire
Blue gems from all these low-iron areas can have vivid “cornflower” violet-blue colors and very low or undetectable magnetic susceptibilities. The rich blue color of these gems is derived primarily from Fe2+-Ti4+ charge transfer rather than from ferrous iron (Fe2+) alone or Fe2+-Fe3+ charge transfer. Darker examples with higher iron and greater magnetic susceptibility can also be found in these regions. However, a diamagnetic (repel) response during floatation shown by a natural blue Sapphire of moderate to strong color points toward an origin of Sri Lanka, Burma, Tanzania or southern Madagascar. A weak response from any dark blue Sapphire also points toward an origin of one of these low-iron regions.
Photo taken at the Los Angeles Museum of Natural History
Vivid Blue Sapphire
The most desirable blue colors are associated with low-iron metamorphic Sapphires, which are mined primarily in Sri Lanka, Burma, Tanzania, and southern Madagascar, with an obscure deposit in Baffin Island, Canada. More than a century ago, metamorphic Sapphires were also actively mined in the now-depleted mines of Kashmir in the Himalayas, where Sapphires set the "gold" standard for vivid blue color.
Sapphire and Ruby
Corundum is the species name for both Sapphire and Ruby. The species is allochromatic (colorless when pure), and magnetic susceptibility is generally low. The difference in the two Corundum varieties lies in the metal impurities that color them. Iron (Fe2+ and Fe3+) and titanium (Ti4+) are the key metals involved in the color of blue Sapphire, while chromium (Cr3+) is the primary coloring agent in Ruby. The complete story of color is very complex, involving a number of trace elements, inter-valence charge transfer, color centers and even silicon (J. L. Emmett et al., 2017). As we look at magnetism in Corundum, we will focus on the primary causes of color.
Inclusions: Star Sapphire gems, as well as star Rubies, are translucent to nearly opaque. Most derive asterism from dense needle-like inclusions of Rutile. Pure Rutile needles are diamagnetic and do not affect the magnetic responses of Corundum. However in the case of Black Star Sapphire, needles of Hematite are responsible for the asterism. In this instance, the iron content of the Hematite results in a strong magnetic response. Black Star Sapphires are the most magnetic of all gem Sapphires, with susceptibilities as high as SI 143.
Black Star Sapphire
Thailand, Strong, (SI 143)
Iron (Fe3+) and color centers invovling Fe3+ are the primary causes of yellow color, and Fe3+ also plays an important role in the color and magnetism of green and brown Sapphire. Orange and Padparadscha (orangey pink) colors are primarily due to small magnetically undetectable amounts of chromium (Cr3+ and color centers involving chromium), although iron can also contribute color and magnetism in these Sapphires. Most natural "fancy color" Sapphires show some degree of magnetic attraction, but in some cases iron content is too low to be magnetically detectable..
We found a number of yellow Sapphires to be diamagnetic. Just a few parts per million of iron (Fe3+) in color centers are sufficient to produce strong yellow color. However, high concentrations of dispersed iron (Fe3+) in Sapphires can also create yellow color and magnetic attraction, as shown below right. Brown and orange Sapphires also can be diamagnetic, and responses range from inert to moderately magnetic depending on iron concentration.
Regardless of geographic origin, green Sapphires on average have higher magnetic susceptibilities than other colors of transparent Sapphires we tested due to relatively high concentrations of iron in the form of Fe3+. We have not encountered any diamagnetic green Sapphires, but magnetic responses still only range from Weak to Moderate (SI 69-135). The green, brown and yellow Sapphires pictured below are all moderately magnetic, with the dark green gem showing the highest magnetic susceptibility of any transparent Sapphire in our study. For comparison, the most magnetic blue Sapphire tested is a dark blue gem from northern Madagascar with a susceptibility of SI 99. The only Sapphires we found to be strongly magnetic are opaque black Sapphires.
Thailand, Moderate (SI 130)
Blue Sapphires from other locations most often have geologic origins that are igneous (volcanic) rather than metamorphic. These higher-iron basaltic gems often show darker “ink” blue colors with gray or black undertones due to significant Fe2+-Fe3+ charge transfer in addition to Fe2+-Ti4+ charge transfer. Dark blue Sapphires from these regions tend to have significantly higher iron content (up to ten times the concentration of low-iron Sapphires) and consequently show greater magnetic susceptibility. Responses to a magnetic wand are often Moderate.
Such gems are found in Australia, Thailand, Cambodia, Viet Nam, Laos, China, Nigeria, Rwanda, Mozambique, northern Madagascar, Colombia, Brazil and Montana. (For more detail on Sapphire origins, see the article "Inside Sapphires" by C.P. Smith). Of course, pale blue and vivid blue gems with lower iron content can also be found in any of these regions. However, as long as the gems are not extremely pale, all blue Sapphires from these areas will likely contain sufficient iron to show some attraction to a neodymium magnet.
Dark Blue Sapphire
Australia, Moderate (SI 61)
Dark Blue Sapphire
Northern Madagascar, Moderate (SI 99)
Treatments: Although it has not yet been studied, high heat treatment of natural Sapphire of any color to enhance color and clarity can theoretically lead to a change in magnetic susceptibility as the valence state of the iron ions is changed from Fe3+ to Fe2+. As Fe3+ iron in blue Sapphire transforms to Fe2+ iron, the amount of charge transfer apparently increases between Fe2+ iron and titanium (Ti4+), and Fe2+-Fe3+ charge transfer may decrease, resulting in brighter and more intense blue color. The dissolution of Rutile needles (titanium oxide) within blue Sapphire during heat treatment may also result in more Ti4+ ions becoming available for charge transfer with Fe2+.
The total amount of Fe2+ iron directly involved in Fe2+-Ti4+ charge transfer remains small and magnetically undetectable. Pictured below, the light blue unheated Sapphire on the left is more magnetic than the darker blue heated Sapphire on the right, indicating that magnetic susceptibility is not necessarily linked to intensity of color. Research on individual Sapphires before and after heat treatment is needed to understand the effects of heat on magnetism, and whether or not any changes in magnetic susceptibility can be detected with a magnet.
Unheated Blue Sapphire
Southern Madagascar, Weak (SI 39)
Heated Blue Sapphire
Sri Lanka, Diamagnetic
Sri Lanka, Moderate (SI 117)
Fancy Color Sapphire
Color Variations: Natural Sapphires are found in a variety of colors other than blue, and these are often referred to in the trade as "fancy" Sapphires. Pink Sapphire is colored primarily by chromium, as is Ruby, but the concentration of chromium is lower in pink Sapphire. The small amount of chromium in pink Sapphire is not magnetically detectable. Purple Sapphire is colored by chromium along with iron and titanium (pink + blue = purple). Color Change Sapphire changes from purple to pink depending on the light source. Color Change Sapphire may contain trace amounts of vanadium in addition to chromium, iron and titanium. As more iron replaces chromium to create purple color in pink Sapphire, magnetic susceptibility increases.
Pink and purple are the least magnetic of all Sapphire colors, with magnetic responses ranging from Inert to Weak. Examples of Sapphires that show Diamagnetic responses (no magnetic attraction) have been encountered among these 2 colors, as well as among some blue Sapphires. As we would expect, colorless Sapphires also show no magnetic attraction.
In our study of over 60 Sapphires and 50 Rubies, we found that both Sapphires and Rubies show magnetic responses that range from Inert (Diamagnetic) to Moderate, with low magnetic susceptibilities ranging from < 0 to 135 SI X 10(-6). In comparison, Spessartine Garnet is on average 100 times more magnetic than Sapphire and Ruby. The Sapphires used for this study come from diverse geographic origins, and include both heated and unheated gems.
Sri Lanka, Weak (SI 56)
Unknown Origin, Moderate (SI 135)
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Macro-inclusions that are magnetic are rarely encountered in transparent Sapphire. The anomalous green Sapphire pictured below has unusually high magnetic susceptibility (SI 191) due to black platelet inclusions in the lower right portion of the gem. A pinpoint wand reveals that the strong magnetic response is restricted to the area of the inclusions. We can speculate that the inclusions may be Hematite.
Yellowish Green Sapphire with Magnetic Inclusions
Sri Lanka, Strong, (SI 191)
White Star Sapphire
Sri Lanka, Diamagnetic
Natural Color-Change Sapphire
Tanzania, Weak (SI 40)
Synthetic Color-Change Sapphire
Synthetic Blue Sapphire
Diamagnetic (no attraction)
Country of Origin: In our study, we encountered diamagnetic blue Sapphires from Sri Lanka, Burma, Tanzania and Baffin Island, Canada. These all share similar conditions of geologic formation (metamorphic compression within a host rock of marble, schist or gneiss) that create low-iron Sapphire. The magnetic responses of blue Sapphires can therefore give us clues about their geographic origins.
Concentration of Iron: In natural untreated Sapphires that come from the same geographic origin, gems generally show increased color intensity as the concentration of iron increases. This correlates with increased magnetic susceptibility from lighter to darker stones, as demonstrated by the unheated Sapphires from Sri Lanka pictured below. The light blue gem is diamagnetic and the dark blue gem is weakly magnetic. Due to low iron content, we occasionally run into natural blue Sapphires that, like synthetic blue Sapphires, show no attraction to a magnet.
This Light Blue Sri Lankan Sapphire is Diamagnetic &
This Dark Sri Lankan Blue Sapphire is Weak (SI < 20)
Natural Sapphire Contains Extra Iron
Vivid Blue Sapphire
Sri Lanka, Diamagnetic
Fluorescence: Sapphires of any color can fluoresce pink or red under long wave UV light if chromium content is high enough and iron content is low enough that it doesn’t quench fluorescence. Sapphires that fluoresce strongly tend to be lighter in color and less magnetic than darker Sapphires with low or no fluorescence and higher iron content, but there is no consistent correlation between fluorescence, tone of color, and magnetic susceptibility. Some diamagnetic Sapphires show no fluorescence, while some moderately magnetic Sapphires can fluoresce strongly due to higher chromium content. The primary determinant of fluorescence in Sapphire appears to be the concentration of chromium (which is not magnetically detectable).
Chromium is found in relatively high concentrations in pink, purple, orange and Padparadshca Sapphires, and these are the colors that fluoresce most consistently and intensely. But chromium can also cause fluorescence in Sapphire when the concentration of chromium is too low to create gem color or significantly modify gem color. We find that blue, yellow, brown and even colorless Sapphire can fluoresce strongly due to chromium. Green Sapphires in our study show little or no fluorescence due to high iron and low chromium content.
Colorless Sapphire, Diamagnetic
Daylight & UV Fluorescence
Blue Sapphire, Weak, SI <20
Daylight & UV Fluorescence
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Cobalt glass is now being used to fill cracks in low-grade colorless and blue Sapphire, creating composite gems with vibrant blue color from gems that would otherwise not be of gem quality. The cobalt glass can cause red fluorescence. Cobalt is also used in surface diffusion of blue Sapphire. It is unlikely that either glass-filling or diffusion treatment with cobalt would contribute to detectable magnetic susceptibility in Sapphire.