Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral 2009, 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.
Colorless Spodumene
LWUV Fluorescence: Strong due to Manganese
Magnetic Response: Inert
Blue Apatite
LWUV Fluorecence: Moderate due to Rare Earth Metals
Magnetic Response: Weak due to Rare Earth Metals
Iron (Fe2+) can quench fluorescence when concentrations are above trace amounts (more than 0.1%). Natural Ruby provides a good example of fluorescence-quenching. Synthetic Ruby contains little or no iron and always fluoresces bright red due to a high concentration of chromium, which can also cause weak magnetic attraction. The intensity of fluorescence in synthetic Ruby can be orders of magnitude greater than in natural Ruby. Chromium is also the chromophore and activator in natural Ruby. Fluorescence can be strong in natural Ruby, but some gems show only weak or moderate fluorescence, and others can appear inert to long wave UV light.
Weaker fluorescence in natural Rubies occurs not only because natural Rubies often contain less chromium than synthetics, but also because they contain a significant amount of iron (Fe3+), which can quench fluorescence. It follows that synthetic Rubies are on average less magnetic than natural Rubies. Increasing concentrations of iron in natural Rubies is associated with greater magnetism and lower fluorescence.
Synthetic Ruby
LWUV Fluoresence: Strong due to Chromium
Magnetic Response: Weak due to Chromium
Natural Ruby
LWUV Fluorescence: Inert
Magnetic Response: Moderate due to Iron & Chromium
Chrome Pyrope Garnet gems are dark red and can contain several times more chromium (4%-8% chromium oxide by weight) than the reddest Rubies, but Chrome Pyrope gems do not fluoresce. This is primarily because Chrome Pyrope has much higher iron content than Ruby. The amount of fluorescence-quenching iron (Fe2+) in these Garnets typically causes a Drag response to a magnetic wand. The lack of fluorescence in Chrome Pyrope is also partly due to a phenomenon known as concentration quenching, which in this case involves an over-abundance of chromium. A high saturation of an activator that normally causes fluorescence can in some cases reduce or eliminate the fluorescence (Roberts, Manuel, 1994). Concentration quenching can occur when the concentration of chromium oxide is greater than 1% by weight.
Fluorescence in Relation to Magnetism
Some gems fluoresce or emit a glow of color when ultraviolet light is applied. The types of metal ions that cause gems to fluorescence are often the same types of metals involved in magnetic susceptibility. Gem fluorescence can be quite striking and beautiful to behold.
To examine fluorescence, we use a long wave ultraviolet (LWUV) flashlight (365nm-375nm wavelength) or a blue laser (405nm), and short wave ultraviolet (SWUV) lamp (254nm). We also use photoluminescence spectroscopy to help us determine the causes of fluorescence in individual gems. With one exception, all photos of fluorescence shown below were taken under long wave UV light.
Cobalt in trace amounts creates blue daylight color and pink or red fluorescence in synthetic blue Spinel, which is magnetically inert. Cobalt, along with iron, also contributes to blue body color in natural blue Spinels, and can create pink/red fluorescence in rare natural Cobalt Spinel gems that contain little iron and a relatively high proportion of cobalt. All natural blue Spinel gems other than Cobalt Spinels are weakly to strongly magnetic due to iron. These typically don't fluoresce, and the cobalt component may not be magnetically detectable. The rare Cobalt Spinel from Viet Nam pictured below derives its pure blue body color and strong pink fluorescence from cobalt. This gem shows weak magnetic attraction when floated, likely due to a small amount of the cobalt activator plus a small amount of iron.
Uvarovite Garnet
LWUV Fluorescence: Inert
Magnetic Response: Large Crystals Drag
Chrome Pyrope Garnet
LWUV Fluorescence: Inert
Magnetic Response: Drags
Yellowish Scapolite
LWUV Fluorescence: Strong due to Iron (Fe3+)
Magnetic Response: Inert
The activator manganese (Mn2+) along with a co-activator such as lead can cause orangey pink fluorescence in colorless Spodumene and pink Spodumene, also known as Kunzite. Both are diamagnetic. Manganese (Mn2+) by itself can create yellow/ fluorescence in some gems, particularly under short wave UV light. Even iron can be an activator of fluorescence (as per the Online Database of Fluorescent Minerals). Iron (Fe3+) in trace amounts induces pink fluorescence in yellow Scapolite, which is a magnetically inert mineral that derives its yellow body color from color centers.
Pink Zircon
LWUV Fluorescence: Strong due to Rare Earths
Magnetic Response: Inert (Diamagnetic)
Colorless Sapphire
LWUV Fluorescence: Strong due to Chromium Magnetic Response: Inert
One of the most common activators that causes LWUV fluorescence in gems is chromium, which induces pink to red fluorescence under long wave UV light. As a coloring agent, chromium can produce either red or green color under visible light. Unlike chromium, vanadium does not cause red fluorescence, and gems colored green primarily by vanadium rather than chromium can sometimes be distinguished by a lack of red fluorescence. High concentrations of vanadium relative to chromium actually quench chromium fluorescence. Chromium is highly fluorescent, and slight gem fluorescence can occur even in daylight. The relatively high concentrations of chromium found in some high-chromium Ruby and Emerald gems can also induce weak magnetic attraction.
But the concentrations of chromium and other activators such as manganese in gemstones are usually lower than what can be detected with a magnet, and often the concentrations are so low that the activators/chromophoes produce no gem color in visible light. As an example, in a number of allochromatic gemstones, chromium ions can cause bright pink fluorescence under LWUV light without producing any red or green gem color in daylight. An example is colorless Sapphire, which is diamagnetic (inert).
Colombian Emerald
Fluorescence: Strong due to Chromium
Magnetic Response: Inert
Various rare earth metals are responsible for fluorescence in a number of gems, as in the blue Apatite pictured below left, which fluoresces pink. Blue Apatite is inert to weakly magnetic due to rare earth elements such as neodymium/praseodymium, which may contribute to fluorescence along with other rare earth elements. The heated pink Zircon shown below right is also colored by rare earth metals. This Zircon fluoresces bright yellow under long wave UV light, likely due to dysprosium, although the rare earths involved are not known for certain. On occasion, uranium acts as an activator in gems, causing green fluorescence, as it does in some rare Opals. Uranium may also be the cause of green phosphorescence in Australian Opals. Uranium is quite magnetic, but due to very low concentrations of this activator, Opals that show luminescence due to uranium are magnetically inert.
Cobalt Spinel
LWUV Fluorescence: Strong due to Cobalt
Magnetic Response: Weak due to Cobalt & Iron
Chrome Tourmaline
Fluorescence: Strong due to Chromium
Magnetic Response: Inert
At times, more than one activator may be responsible for fluorescence within a single gem, as we find in some synthetic Spinels (see Spinel pg.5). As another example, the weakly magnetic colorless “Leuco” Grossular Garnet pictured below fluoresces orangey pink due to a combination of chromium and manganese. Chromium acts as an activator and not a chromophore in this natural gemstone, causing pink fluorescence under long wave UV light. The manganese activator reveals itself as orange fluorescence when the gem is viewed with a blue laser, and as yellowish green fluorescence under short wave UV light.
Chromium, vanadium and manganese are present in concentrations too low to contribute to the weak magnetic attraction of this gem. The very low magnetic susceptibility we detect with a magnetic wand is due entirely to a small amount of cryptic iron (Fe3+), which is neither an activator nor a chromophore in this gem.
Fluorescence in the colorless Grossular Garnet above can be analyzed with a spectrometer when we use a UV light source rather than incandescent light. This is called fluorescence spectroscopy, and it can help us identify which metal activators are present. With this type of spectroscopy, we examine the light that is transmitted by activators rather than the light that is absorbed by chromophores. On the transmission spectrum graph below for the colorless Grossular Garnet, the long wave UV transmission peaks of manganese in the yellow/orange portion of the light spectrum, as well as peaks indicating chromium transmission in the red portion of the spectrum, are clearly evident.
Colorless Grossular Garnet
Photoluminescent Transmission Spectrum for Colorless Grossular Garnet
405nm Excitation
Our studies reveal that fluorescence is almost always restricted to allochromatic gemstones, which are less magnetic than idiochromatic gems. The primary cause of the fluorescence is metal ion impurities, most commonly chromium and manganese. Fluorescence often involves the same paramagnetic transition metals and rare earth metals that can cause magnetic attraction, absorption spectra, Chelsea filter reactions and gem color. When these metals cause fluorescence, they are referred to as activators.
Metallic activators are often found only in trace amounts (less than 0.1% by weight) within gems, and although they may be detected via fluorescence, concentrations are usually too low to induce any visible magnetic attraction when the flotation method is used. As examples, the light green Colombian Emerald above and dark green Chrome Tourmaline pictured below both show green body color and pink to red fluorescence due to chromium. Because both gemstones have a low concentration of chromium (likely less than 0.4%) and contain no magnetically detectable iron (under 0.1%), they are magnetically inert (diamagnetic).
A good example of concentration quenching is found in Uvarovite Garnet, which is an idiochromatic gem colored dark green by high chromium content. Whether in druse form or as large individual crystals, Uvarovite does not fluoresce. This Garnet species is quite magnetic, but not due to iron or manganese. Large individual Uvarovite crystals show a Drag response to a magnetic wand due almost entirely to an exceptionally high concentration of chromium (up to 27% chromium oxide by weight). This level of chromium inhibits all fluorescence. Perhaps Concentration quenching is the reason manganese (Mn2+) does not cause fluorescence in idiochromatic gems such as Rhodochrosite and Rhodonite, which both show a Pick-up magnetic response due to high manganese.