Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Natural Magnetite Crystals (Pick Up)
Black Star Diopside (Picks Up)
pale gem is weakly magnetic,
dark gem is strongly magnetic
Strong color does not always indicate that a gem contains a high concentration of metal impurities and is strongly magnetic. In some cases, gems that are vibrantly colored by metal ions can be diamagnetic, as shown by the 3 examples below. Very small concentrations of metals can sometimes be sufficient to create these intense colors, as seen in the bright blue of synthetic Spinel (due to cobalt). Cobalt ions are strongly paramagnetic, but the gem below (left) shows no attraction to a magnet because the concentration of cobalt ions is too low to be detectable. In Chrome Tourmaline (center), the lack of attraction is due to low concentrations of the coloring agents chromium and vanadium. In Sphalerite (right), vivid orange color is due to iron involved in a color process called inter-valence charge transfer (see description below). Gems colored by charge transfer are often diamagnetic due to low concentrations of metal ions, as is the case with Sphalerite.
Chrome Tourmaline (Inert)
Synthetic Spinel (Inert)
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Valence State: Besides the concentrations of metal ions and their varying magnetic susceptibilities, the valence state of these ions is also a significant factor that affects the degree of magnetism in gems. It also affects color. Valence refers to the gain or loss of electrons by an atom, resulting in an affinity to bond with other atoms. As an example, iron ions within gems can exist in 2 different valence states: ferrous iron (Fe2+) or ferric iron (Fe3+). In Beryl gems, ferrous iron (Fe2+) is responsible for higher magnetic susceptibility than ferric iron (Fe3+). Gem treatments such as heat and irradiation can change the valence states of these metal ions that cause color in gems.
Idiochromatic or Allochromatic: This is related to concentration. Attraction to a magnet tends to be greater in idiochromatic gems such as Garnet and Peridot (colored by iron), and gems such as Rhodocrosite and Rhodonite (colored by manganese). These gems are self-colored, meaning color is due to metals that are part of the inherent chemistry of the gem. The chemical formula of a gem species tells us whether or not it is idiochromatic. The formula will include one or more metals that can cause color and magnetic susceptibility. Any natural transparent gemstone larger than 0.5ct that is magnetic enough to be picked up by a magnet is an idiochromatic gem. Garnet and Peridot are the only idiochromatic minerals that are commonly faceted as gems.
Of all the primary gemstones, Spessartine Garnet is the most strongly magnetic, and this is due to a high concentration of manganese. After Garnets, Peridot is the most magnetic transparent gemstone, due to iron. One of the most magnetic transparent secondary gemstones that this researcher has tested is Axinite (Ferroaxinite species). This mineral is also idiochromatic. Due to its high iron content, Axinite is more magnetic than Peridot, yet less magnetic than Spessartine and most other Garnets.
Snowflake Obsidian (Drags)
Mahogany Obsidian (Picks Up)
Opaque gemstones, whether allochromatic or idiochromatic, can be exceptionally magnetic. Obsidian, Jasper and black Tourmaline all show very strong magnetic suseptibility due to high concentrations of iron.
Idiochromatic Spessartine Garnet
Allochromatic Indicolite & Verdelite, & Yellow Tourmaline (all Drag)
As we have shown for Quartz, Diamond and Topaz, colorless gems of any kind can potentially show magnetic attraction if metallic inclusions are present in sufficient size or concentration. For information about how to test individual inclusions within gems in order to determine if the inclusions are the cause of magnetism, refer to the section titled Magnetic Inclusions at the bottom of page 4 of How To Use a Magnet for Gem Identification.
There is one opaque variety of gemstone that always contains magnetic metallic inclusions as an identifying characteristic. This is opaque Black Star Diopside, the most strongly magnetic gem material of all the primary gemstones. This gem is picked up by a magnet. Needle-like inclusions of Magnetite are thought to be responsible for the asterism in Black Star Dopside.
However, there are a couple of gems in the Feldspar group to look out for: Oligoclase ("Confetti" stone or "Sunstone" variety) and Labradorite ("Spectrolite" variety). Geologists report that Feldspar is often a host rock for Magnetite. The orange confetti-like inclusions in Oligoclase sunstone are composed of thin flecks of Hematite, which are too small and thin to induce magnetic attraction. However, spots of black Magnetite can also be present, and these can induce visible magnetic attraction. Our second example, Spectral Labradorite, is iridescent due to surface diffraction of light. Spectral Labradorite is usually not magnetic, but when it contains microscopic particles of magnetite dispersed throughout the gray body of the gem, a magnetic response may be apparent. Depending on the concentration of Magnetite, such responses can range from weak to strong.
Black Tourmaline in Quartz
Concentration of Metals: Magnetism in gemstones is not only determined by the types of metal ions present, but also the concentration of those ions. Colorless and near-colorless gems typically show no magnetic attraction because they lack sufficient metallic chromophores. Gems that are darker in color often have higher concentrations of metal impurities and are more magnetic than pale stones of the same type. For example, as Chrysoprase color becomes more saturated due to higher nickel content, these gems become more magnetic. Chrysoprase responses can vary widely from inert to strong. Such correlation is also clearly seen in blue Aquamarine Beryl, where increasing intensities of color and magnetic response are due to increasing concentrations of iron. Near-colorless Aquamarine can be inert.
Magnetic Inclusions: Large macroscopic mineral particles of a size that can be detected with a magnetic wand are seldom encountered as inclusions within transparent gemstones. Quartz may be an exception, as many different types of mineral inclusions can be found in relatively large sizes within Quartz. When free of inclusions, Quartz is diamagnetic, but mineral inclusions visible to the naked eye can induce a magnetic response characteristic of the mineral. The most common example is Tourmalinated Quartz, which is most often seen as black Tourmaline within Quartz, and more rarely as blue Tourmaline in Quartz. Black or blue Tourmaline inclusions can make Quartz gems weakly magnetic. Translucent to opaque Quartz such as Tiger's Eye Quartz and Pietersite Chalcedony Quartz can show strong magnetic attraction due to Hematite inclusions.
Spessartine Garnet Inclusion in Topaz
On rare occasions, inclusions of metallic particles or compounds containing ferromagnetic/ferrimagnetic metals can cause a "false" or unexpected magnetic response when we test a gem with a magnet. These inclusions may appear as black particles of iron compounds such as Magnetite or some other mineral. Magnetite itself holds a magnetic charge, like a weak permanent magnet.
In a feature article in Gems and Gemology (Winter 2012), we published the only known report of gem-grade natural Diamond showing a magnetic response due to natural inclusions (probably pyrrhotite, a black sufide mineral). We have also encountered a weakly magnetic Beryl gem in which green color and magnetism are due to unidentified green inclusions deposited inside growth tubes. Such cases of magnetic inclusions are unusual and rare to find in transparent gems.
With Magnetic Inclusions
With Magnetic Inclusions
Hematite in Pietersite (Chalcedony Quartz)
pale gem is inert,
dark gem is strongly magnetic
Factors that Affect Magnetism in Gems
Inter-valence Charge Transfer: This is a color process that can create or alter color in gemstones that contain transition metals. Strong color can be produced even when the metals are in very low concentrations. Color arises when electrons jump from one transition metal to another. Inter-valence charge transfer in gems can be from iron to iron (Fe2+-Fe3+), iron to titanium (Fe2+-Ti4+), and manganese to titanium (Mn2+-Ti4+). Iron involved in iron to titanium (Fe2+-Ti4+) charge transfer does not induce detectable magnetic susceptibility in gems, probably because the concentration of iron involved is too low. Charge transfers between other metal ion pairs such as Fe2+-Fe3+ may affect magnetic susceptibility differently, but this needs to be researched. The photos below are examples of 3 gems that derive their colors from iron-titanium (Fe2+-Ti4+) charge transfer : a blue Sapphire from Sri Lanka, an orange Dravite Tourmaline, and an Andalusite.
Theory vs. Practical Application: Exactly how various inter-valence charge transfer processes quantitatively affect magnetic susceptibility in gemstones has not been researched. Valence Chemistry in gemstones and color processes such as inter-valence charge transfer are aspects of Crystal Field Theory and Ligand Field Theory. These are hugely complex subjects that are often not thoroughly understood even by the scientists who specialize in those areas of research. Fortunately, we can use a magnet to identify gems without knowing anything about the types of metals, valence states or color processes involved in those gems.
Blue Sapphire (Sri Lanka, Inert)
Dravite Tourmaline (Inert)
As an example, let's look at the effects of valence changes in Beryl, as seen in the 2 large and beautiful gems pictured below. The color of Green Beryl is a blend of blue color due to ferrous iron (Fe2+) and yellow color due to ferric iron (Fe3+). On average, Green Beryl shows weaker magnetic responses than Aquamarine, since Green Beryl contains less ferrous iron (Fe2+) and more ferric iron (Fe3+). Blue Aquamarine is colored primarily by ferrous iron (Fe2+), and is more magnetic. The magnetic responses of Green Beryl range from Inert to Weak, while Aquamarine responses range from Weak to Moderate.
Heat treatment is used to remove the yellow color in Green Beryl (as well as in blue-green Aquamarine) in order to change the color to a purer blue that is now considered more desirable for Aquamarine. Heat treatment changes gem color by changing the valence state of the iron ions within these gems from Fe3+ to Fe2+. This increases the magnetic susceptibility of the treated gems, although research on individual Beryls before and after heat treatment would be needed to verify this assumption.
Unheated Green Beryl (Weak) & Heated Aquamarine Beryl (Moderate)
The bar graph below compares the magnetic susceptibility (or "degree of magnetism") of 10 allochromatic gemstones compared to the magnetic susceptibility of 5 idiochromatic gemstones. These gems are ordered from least magnetic to most magnetic. The height of each colored bar represents the maximum magnetic susceptibility we were able to measure for each type of gemstone. The most magnetic idiochromatic gemstone shown on the graph (Rhodochrosite) is 60 times as magnetic as the least magnetic allochromatic gemstone (Emerald).
A Comparison of Magnetic Susceptibilities of Allochromatic vs. Idiochromatic Gems
Higher concentrations of transition metals within a species not only increases the measured magnetic susceptibility (degree of magnetic attraction, represented as SI), but also increases refractive index (RI) and raises density (specific gravity, SG). In natural stones, this is most noticeable in idiochromatic gems such as Garnet. For example, the refractive index of Almandine Garnets can increase from RI 1.773 in a gem with magnetic susceptibility of SI 2200 (below left) to RI 1.795 in a gem with a higher magnetic susceptibility of SI 2600 (below right). Color intensity is not affected here.
Almandine Garnet (low RI 1.773)
Pink YAG (Anomalously High SG 5.7)
Types of Metals: Some metals have much stronger magnetic susceptibilities than others. Iron and manganese ions in gemstones are strongly magnetic and are often easily detectable. An example is strongly magnetic orange Spessartine Garnet, colored by manganese. Chromium and vanadium ions have lower magnetic susceptibilities and occur in lower concentrations, and these metals are rarely detectable with a magnet in gemstones. As an example, blue Tanzanite colored by vanadium is diamagnetic.
Because metallic coloring agents like chromium and vanadium can be magnetically undetectable in gems, the magnetic response shown by a gemstone is not necessarily related to the gem's color. Often more than one type of transition metal is present within a single gem. As we will see, traces of iron impurities may contribute to gem’s magnetic response without contributing to its color. The types of metals that cause color in gems are discussed in detail on the next page.
Spessartine Garnet (Picks Up)
Any change in color that results from the heat treatment of any gem, whatever the gem species, is always due to changes in the valence states of the metal ions that cause gem color. Changes in magnetic susceptibility due to heat treatment are likely small and may not be noticeable with a magnetic wand. For an example of how heat treatment can affect the valence states and magnetic susceptibilities of Sapphire gems, see the Sapphire & Ruby page.
For information about how the Floataion method can be used to detect variations in magnetic susceptibility, see page 2 of How to Use a Magnet for Gem Identification.
Most Tourmaline gems belong to the Elbaite species, which is considered allochromatic. Due to the relatively high iron content found in blue Indicolite Tourmaline and green Verdelite Tourmaline, these are the most magnetic of all transparent allochromatic gemstones. They are dragged by an N-52 magnet. Yellow Tourmaline can be equally magnetic due to manganese. Brown Dravite Tourmalines are not attracted to a magnet due to low iron content. For more details about color and magnetism in Tourmalines, see the 3-page section titled Tourmalines.
Peridot is a variety of the Forsterite species within the Olivine group. Although Peridot is generally referred to by gemologists as an idiochromatic gem, 90% of its composition is allochromatic Forsterite, which is colorless when pure. Peridot contains only about 10% Fayalite, an idiochromatic species containing iron that produces green color and magnetic attraction. If classification were consistent in gemology, Peridot would be correctly referred to as an allochromatic gem, as is Elbaite Tourmaline, Grossular Garnet and Pyrope Garnet. All of these gems are colorless when pure, with color and magnetic attraction due to mixing with idiochromatic species within their own mineral group. Perhaps it would be most accurate and informative to refer to all of these gems, including Peridot, as a blend of idiochromatic and allochromatic species, with color and magnetism produced by the metal ions of the idiochromatic component(s).
Peridot Composition is Primarily Allochromatic
The vast majority of gemstones are allochromatic. They tend to show weaker magnetic responses than idiochromatic gems because allochromatic gems generally have lower concentrations of metallic chromophores. These coloring agents exist only as impurities that are not essential to the gem's chemistry. When pure, all natural allochromatic gems are colorless and diamagnetic.
Blue Sapphires (such as the one pictured above left) that show no magnetic attraction derive all of their color from Fe2+-Ti4+ charge transfer, but most blue Sapphires contain additional iron and are not diamagnetic (see the Sapphire & Ruby page). Dravite Tourmalines colored primarily by iron-titanium charge transfer are diamagnetic. Some gems that are colored by inter-valence charge transfer can show weak magnetic attraction, such as the Andalusite above (right) and the dark blue Sapphire pictured below. In these cases, magnetic attraction is due to additional iron ions that are not involved in the iron to titanium charge transfer process. Titanium ions by themselves (Ti4+ and Ti3+) do not cause magnetic susceptibility in gems.
This Dark Blue Sapphire is Weakly Magnetic
Yttrium Aluminum Garnet (YAG) is a synthetic Garnet that is allochromatic, with the rare earth metals erbium and holmium added during synthesis to create pink color. The intense pink YAG pictured below has such a high concentration of erbium that its level of magnetic susceptibility reaches SI 5200, higher than magnetic susceptibility of any natural Garnet. The density we look for to identify YAG gems is about SG 4.5, but the unusually high erbium content in this gem raises the density to SG 5.7, mimicking the high density of Cubic Zirconia. This is because the density of erbium metal itself is high (SG 9.1). In allochromatic gems, the densities of all transition metal and rare earth metal impurities are higher than the densities of the gems they color, and therefore increasing concentrations of these metals increase the refractive index and density of the gems.
Almandine Garnet (higher RI 1.795)
Allochromatic vs. Idiochromatic
Topaz is another transparent diamagnetic mineral species that can contain sizable inclusions that are attracted to a magnet (such as inclusions of Garnet or Tourmaline), but we rarely encounter these in faceted Topaz gems. The Topaz shown below contains a red Spessartine Garnet inclusion, and it is the first reported case of a Topaz showing visible magnetic attraction (due to the inclusion).
Blue Tourmaline in Quartz
Effects of valence on magnetic susceptibility can be related to concentration of metals. For example in Tourmaline, manganese in the valence state of Mn3+ causes red color, but only very low concentrations of Mn3+ are needed to create strong color. Red Tourmaline is inert to weakly magnetic. In contrast, manganese as Mn2+ is responsible for yellow color in Tourmaline. High concentrations of Mn2+ are often found in yellow Tourmaline and Paraiba Tourmaline, and these Tourmalines are often strongly magnetic.
Red Tourmaline (Mn3+), Weak
Yellow Tourmaline (Mn2+), Drags