Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Spinel: Color and Magnetism
The magnetic responses of Spinels vary widely, but the majority of gems are weakly to moderately magnetic. We studied magnetism in 117 natural Spinel gems in a range of colors: gray, green, blue, violet, purple, pink, orange & red. Most of the Spinels originated from the established mining areas of Sri Lanka and Burma, but a few originated from less common localities: Tanzania, Viet Nam and Tajikistan.
All blue & red Spinels in our study showed some magnetic attraction, and on average, blue Spinels were twice as magnetic as other colors. A magnetic wand allowed us to easily distinguish natural blue & red Spinels from most synthetic blue & red Spinels, as the synthetics are almost always diamagnetic.
Natural Blue and Red Spinel
What is Spinel?
The type of Spinel that is fashioned into gemstones is magnesium Spinel, a magnesium aluminum oxide mineral that is singly refractive, with a refractive index near 1.72. Gem Spinel is allochromatic, meaning it is colorless when pure (i.e. the magnesium content is unadulterated). Gem Spinel is actually part of a large group of minerals that includes not only transparent Spinel, but also many other mostly opaque idiochromatic Spinel species whose chemistry often blends with gem Spinel in a solid solution series. A few species in the Spinel Group include:
Spinel (magnesium aluminum Spinel, MgAl2O4)
Gahnite (zinc aluminum Spinel, ZnAl2O4),
Hercynite (iron aluminum Spinel, FeAl2O4)
Chromite (iron chromium Spinel, FeCr2O4)
Cochromite (cobalt, nickel, iron, chromium, aluminum Spinel, (Co,Ni,Fe) (Cr,Al)2O4)
Magnetite (iron oxide, Fe3O4)
Because all of the above species except magnesium Spinel (gem Spinel) contain high concentrations of transition metals as part of their native chemistry, they have high densities and high refractive indices. The species that contain iron are highly magnetic. Magnetite is a black opaque Spinel that itself acts as a natural magnet, and may have the highest magnetic susceptibility of any naturally-occurring mineral. These Spinel species have a cubic crystal structure, and they often form octahedral crystals that grow out of matrix in the shape of 4-sided pyramids.
Red Spinel Crystals
Transparent gem Spinel is one of the few types of gemstones that is seldom heated, treated or enhanced. However, in recent years heat treatment of pink and red Spinels to improve clarity has been reported, as well as cobalt diffusion of blue Spinels to improve color (Peretti et. al., 2015). Treatment is uncommon.
Gahnite, also called Zinc Spinel, is another Spinel species that can be transparent, but Gahnite is an uncommon mineral and rarely seen as a gemstone. It has a high refractive index (1.79-1.82) due to the high concentration of zinc in its chemical makeup. Cut gems are typically dark green, and can appear black in reflected light. The green color is not due to the transition metal zinc, but instead to iron-iron (Fe2+-Fe3+) charge transfer. Blue Gahnite crystals colored by traces cobalt have also been found in Nigeria. Gahnite can be strongly magnetic, again not because of zinc, which is diamagnetic. The magnetism is due to iron content.
Gahnite mixes with gem Spinel in solid solution series, with zinc ions replacing magnesium ions, mostly in very small amounts. Zinc ions do not generate magnetic responses or create color, although they seem to be associated more with blue Spinel than red Spinel. Rare dark blue Spinels are called Gahnospinels when they are intermediate in composition between Spinel and Gahnite, but most dark blue gem Spinels are not Gahnospinels. Some rare light blue Spinels can also contain a high enough concentration of Gahnite (Zinc) to qualify as Gahnospinels. Because zinc raises refractive index, Gahnospinel is easily distinguished from other blue Spinels by its higher refractive index (1.735-1.79). No Gahnospinels intermediate between Spinel and Ghanite were tested in this study. However, a single faceted green Gahnite Spinel was examined for magnetic susceptibility. It is shown below in reflected light and transmitted light.
Gahnite in Transmitted Light
Shows Bright Green Color
Cause of Magnetism
In most cases, magnetism in natural gem Spinel of any color is due entirely to iron. All natural blue and red Spinels in this study showed some degree of magnetic attraction. Stronger magnetic responses and higher measured magnetic susceptibilities corresponded to higher iron content. In some of the red Spinels we tested, chromium content may possibly be great enough (greater than approx. 0.4%) to contribute a bit to the magnetic responses and measured magnetic susceptibilities. Theoretically, some high-chromium low-iron red Spinels could possibly owe all of their weak magnetic susceptibility to chromium content, but spectrometer analysis indicate that this is not the case in any of the red Spinels tested in our study.
In red Spinels and natural Spinels of any color that show detectable magnetism, analysis with a UV-Vis-NIR spectrometer invariably indicate the presence of iron in addition to cobalt and/or chromium. With the exception of perhaps one Cobalt Spinel sample, cobalt in blue gems was almost certainly in concentrations too low to contribute anything to magnetic susceptibility. However, low concentrations of cobalt do partly contribute to blue color.
Blue Spinel, Sri Lanka
4.24ct., Weak, SI 61
Red Spinel, Sri Lanka
3.67ct., Weak, SI < 20
The primary coloring agent in most blue Spinels is iron (ferrous iron, Fe2+), as it is in some other gem species such as Aquamarine of the Beryl species and most blue Tourmalines of the Elbaite species. Iron is the reason all natural blue Spinels tested in this study showed magnetic attraction. All natural Spinels that had a component of blue color or gray color, including purple and violet Spinels, were also magnetic to some degree. In addition to iron, cobalt is also present in blue Spinels, mostly in trace amounts (<.1% cobalt oxide by weight). The presence of cobalt in the gems we tested was confirmed by spectrometer analysis.
Researchers have shown that these trace amounts of cobalt are enough to contribute to some degree to the color of blue Spinels (Shigley & Stockton, 1984), but such low concentrations of cobalt do not contribute to magnetic attraction. An exception may be Cobalt Spinel, which is colored primarily by cobalt rather than iron. Cobalt Spinel will be discussed separately below.
The color saturation of blue Spinels varies from light to dark, and color varies from gray-blue to green-blue to pure blue. All light blue Spinels in this study (like the one pictured below left) were weakly magnetic (SI 35-69) due to low concentrations of iron. Gray tones, which are common in blue Spinels (shown below right) and Spinels of other colors, are the result of ferrous (Fe2+) iron (Chauvire et. al., 2015).
Light Blue Spinel, Sri Lanka
1.95ct., Weak, SI 35
Light Gray-Blue Spinel
1.49ct., Weak, SI 42
Darker color does not necessarily correspond to greater magnetic susceptibility, as illustrated by the blue Spinels compared above and below, all weakly magnetic. The 2 gems of each pair have similar magnetic susceptibilities, but the gems on the right (above right and below right) are significantly darker. Darker blue color is often the result of iron-to-iron (Fe2+-Fe3+) intervalence charge transfer, which is not dependent on higher iron levels. This color process can also add a green component to the blue color (as seen in the gem below right).
Medium Blue Spinel, Burma
3.79ct., Weak, SI 74
Dark Green-Blue Spinel, Sri Lanka
2.49ct., Weak, SI 69
Increasing concentrations of both ferric iron (Fe3+) and ferrous iron (Fe2+) together leads to purer blue color (D'Ippolito et. al., 2015). This also results in stronger magnetism. The purer-blue Spinel shown below (left) has 3 times the magnetic susceptibility as the gray-blue gem (right). The gray-blue gem on the right has lower total iron content and presumably a higher percentage of Fe2+ relative to Fe3+.
Medium Blue Spinel, Sri Lanka
2.66ct., Strong, SI 187
Dark Gray-Blue Spinel, Sri Lanka
2.92ct, Weak, SI 61
The color of the violet-blue Spinel below may also be related to a dominance of ferrous iron (Fe2+) over ferric iron (Fe3+). This gem was sold as a Cobalt Spinel, but this is not correct. Spectrometer analysis suggests that some cobalt is detectable, but the dominant coloring agent is iron.
Violet Blue Spinel, Sri Lanka
4.08ct., Moderate, SI 87
All blue Spinels in this study that were strongly magnetic also had dark blue color saturation. None were light blue. But as already mentioned, not all dark blue gems are strongly magnetic. Iron to iron (Fe2+-Fe3+) charge transfer can darken a gem without increasing magnetic susceptibility.
The over-dark pure blue gem shown below had by far the highest magnetic susceptibility of any Spinel tested. This Spinel showed a Drag response (SI 360), presumably due to an unusually high concentration of both ferric iron (Fe3+) and ferrous iron (Fe2+).
This anomalous Spinel was sold incorrectly as a Gahnospinel, which has a composition intermediate between zinc Spinel (Gahnite) and magnesium Spinel. However, the refractive index of 1.722 identifies this gem as magnesium Spinel. Darker blue color in Spinel may or may not be accompanied by increasing concentrations of cobalt. Cobalt levels are difficult to assess with a spectrometer, Chelsea filter or UV fluorescence, as the presence of cobalt is almost entirely masked by iron in all blue Spinels other than Cobalt Spinels.
Very Dark Blue Spinel, Sri Lanka
1.76ct., Drags, SI 360
Natural or Synthetic?
Any magnetic attraction in blue Spinel is generally diagnostic for natural origin (as opposed to synthetic origin). Most blue synthetic gems found in the marketplace are produced by the flame fusion process, and they are colored by just a trace amount of cobalt that is enough to create strong blue color. This small amount of cobalt is magnetically undetectable, as it is in most natural blue Spinels. Synthetic blue Spinels are not manufactured with iron as a dopant. Natural blue Spinels are mostly colored by iron, and all contain magnetically detectable iron.
As an example, the unusually large size (10.87ct.) of the dark blue Spinel pictured below makes us suspect that it could be lab-grown. Additionally, microscopic examination reveals no definitive natural inclusions. How can we determine whether it is synthetic or natural? Magnetic testing with floatation reveals that the gem is moderately magnetic, something we never encounter in synthetic blue Spinels. This simple test tells us the gem is natural. Iron can also be detected in this gem with a spectrometer, confirming it's natural origin.
Had this dark blue gem been synthetic, a cobalt spectrum would have been visible, even with a hand-held spectroscope. Synthetic blue Spinels are also easily distinguished by pink fluorescence under long wave UV light due to cobalt in the absence of iron. Iron in natural Spinel quenches any fluorescence. Most synthetic Spinels (flame-fusion type) also have a slightly higher refractive index (approx. 1.73) than natural Spinels (approx. 1.72).
Natural Dark Blue Spinel, Sri Lanka
10.87ct., Moderate, SI 95
Cobalt Spinels are rare, and most blue Spinels that are marketed as Cobalt Spinels or “cobalt blue” Spinels are actually common blue Spinels colored primarily by iron. When cobalt ions replace magnesium ions in Spinel, they produce only pure blue color rather than gray-blue or green-blue. Depending on the concentration of cobalt, color saturation can theoretically range from light blue to vivid blue. No ratio of iron to cobalt has been established to define what makes a true Cobalt Spinel, but logically, cobalt must either be the dominant cause of color or an equal contributor to color.
Pure blue color in a natural Spinel is a sure sign that the gem is a genuine Cobalt Spinel. But whether iron of cobalt is the dominant chromophore may not always be obvious simply by looking at the color, or by checking the absorption spectrum with a hand-held spectroscope. Analysis with a spectrometer is a better means to determine whether or not cobalt is primarily responsible for the blue color. Cobalt shows slightly different absorption features on a spectrometer graph than does iron.
High cobalt content also causes Spinel to appear bright pink under a Chelsea filter, or even bright red when the concentration of cobalt is high enough. Cobalt Spinels also fluoresce pink or red under long wave UV light. The body color in the vivid pure blue Spinel pictured below is a clear indication that cobalt is the primary coloring agent. Such vivid cobalt color suggests low iron content and low magnetic susceptibility. Unfortunately, the rare gem below was not one of our study samples.
Cobalt Spinel, 2.59ct., Viet Nam
Photo Courtesy of Palagems.com
Several Cobalt Spinels from Luc Yen, Viet Nam were tested in our study, including a number or rough crystals.
The vivid blue Cobalt Spinel on the left is colored predominantly by cobalt, but iron is also detectable with a spectrometer. This Cobalt Spinel is weakly magnetic (SI 55). This gem glows bright pink under a Cheslea filter, and fluoresces a strong pink under long wave UV light, both due to cobalt. The lighter Cobalt Spinel on the right is less fluorescent and slightly more magnetic (SI 73), suggesting less cobalt and more iron content.
Cobalt Spinel, Viet Nam
0.25ct, Weak, SI 73
Cobalt Spinel, Viet Nam
0.03ct., Weak, SI 55
Exactly which portion of the magnetic susceptibilities of the Cobalt Spinels above is due to cobalt and which to iron is difficult to say. Cobalt oxide is equally as magnetic as iron oxide, but approximately 20 times less cobalt than iron is needed to produce blue color. We suspect that the small amount of cobalt in the examples above is barely detectable with a magnetic wand when the floatation method is used, and the cobalt content could be too low to be measurable with a Hoover balance (SI < 20). Any measurable magnetic susceptibility is likely due to iron. In the 2015 study by Boris Chauvire et. al., a natural Cobalt Spinel from Viet Nam with saturated blue color contained only 1236 parts per million cobalt (approx. 0.16% cobalt oxide), which would likely be high enough to create a weak magnetic response and perhaps contribute some portion to measurable magnetic susceptibility.
Natural spinel that has been diffused on the surface with cobalt has pure blue color and can look similar to genuine cobalt spinel. And like untreated cobalt spinel, cobalt-diffused spinel gems appear red under a Chelsea filter and fluoresce red under longwave UV light. They also show a pronounced cobalt spectrum under a spectroscope. The gem shown below is weakly magnetic, most likely due to natural iron, as the amount of cobalt used in diffusion is very small.
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Dark Blue Spinel
1.44ct, Strong, SI 152
All color and magnetism in natural gem Spinel is derived from the non-gem Spinel species that are in solid solution series with it. In other words, color and magnetic susceptibility are due to a small degree of inter-species mixing. For example, iron can mix into gem Spinel to create blue and gray colors when colorless magnesium ions are partly substituted by iron ions derived from Hercynite Spinel. Chromium ions from Chromite Spinel can substitute for aluminum ions in gem Spinel to produce red color. Cobalt ions from Cochromite can substitute for magnesium ions in gem Spinel to create blue color.
This exchange of these chromophores within Spinel minerals is illustrated by the Hercynite specimen pictured below. In addition to the mostly black color from iron (Hercynite), we can see pink color from chromium (Chromite) and blue color possibly from cobalt (Cochromite). The white color is likely marble matrix.
Causes of Color
The complete story of color in gem Spinel is complex, and even though researchers have studied the subject for years, some questions remain. Fundamentally, Spinel color is created by 3 transition metals: iron, chromium and cobalt. These metal ions replace the colorless magnesium ions in gem Spinel. Iron creates blue and gray colors, and iron is the source of the gray "mask" that is ubiquitous in Spinel. Cobalt creates only blue color. Red and pink color in Spinel is created when chromium ions replace the aluminum ions. Combinations of these 3 metals in varying ratios and concentrations produces non-primary colors such as purple and orange. Intervalence charge transfer between ferrous iron (Fe2+) and ferric iron (Fe3+) contributes green color (D'Ippolito et. al., 2015), although Spinels with green as the primary body color are rare. Natural yellow Spinel is extremely rare and has only recently been found, although synthetic yellow Spinel has been created in China.
Hercynite Specimen, Eifel, Germany
with Iron, Chromium and Cobalt
Gahnite, 0.57ct, Brazil, RI 1.795
Strong, SI 218,Transmitted Light
The unusual dark blue Spinel of unknown origin shown below is a Color Change Spinel, changing from blue to purple in incandescent light. Chromium content is likely responsible for the color change. But this gem shows no chromium or cobalt fluorescence under UV light, probably due to the quenching effect of iron. Spectrometer analysis and magnetic susceptibility shows that this gem has high iron content. The magnetism (SI 200) is one of the highest recorded for blue Spinel.
Color Change Spinel with Cobalt
Daylight & Incandescent Light
4.78ct, Strong, SI 200
The two rough Spinel crystals pictured below are also Cobalt Spinels. The crystal on the left is weakly magnetic and fluoresces bright red, suggesting high cobalt content. The slight purplish tone could be due to a bit of chromium (red) mixing with the cobalt (blue), although this is difficult to know for sure even with a spectrometer. The blue octahedron on the right fluoresces pink rather than red, suggesting less cobalt than the crystal on the left, and the octahedron is a little more magnetic, suggesting more iron.
Cobalt Spinel Rough, Viet Nam
Cobalt Spinel Rough, Viet Nam
Contents of this Section:
Pg. 1) Overview, Blue Spinel, Cobalt Spinel