Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Almandine Garnet Absorption Spectrum (image courtesy of John S. Harris)
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Spectroscopy in Relation to Magnetism
Spectroscopy and the study of magnetism in gemstones are 2 sides of the same coin. Hand-held spectroscopes used for gem identification detect the presence of metal ions that cause color, just as magnets do. The bands of absorption seen with a spectroscope are indicators of exactly the same metal ions that cause magnetic attraction. The locations of the black absorption bands along the light spectrum provide clues about which particular metals are present within a gem.
In many cases, gems that show prominent absorption patterns also show a significant degree of magnetic attraction. As you might expect, gemstones that show no visible absorption pattern also tend to show little or no attraction to a magnet. The strong absorption spectrum shown below is due to high iron content in Almandine Garnet.
With a spectroscope, we are better able to detect metals that are strong chromophores such as chromium (Cr3+) and cobalt (Co2+) than we can with a magnetic wand. But a magnet allows us to detect weak chromophores such as iron (Fe3+) and manganese (Mn2+) at lower concentrations than are detectable with a spectroscope. Without our magnetic wand, more sophisticated instrumentation such as a spectrometer are required to detect low levels of iron and manganese. You can find more information about spectroscopy in the book A Student's Guide to Spectroscopy by Colin Winter (2003) OPLspectra.com, The Spectroscope and Gemmology by Basil Anderson, and at John Harris' gem spectroscopy website gemlab.co.uk.
Hand-held OPL Teaching Spectroscope
Only manganese is responsible for the strong magnetism of this gem, but magnetic testing alone cannot reveal this. Magnetic susceptibility only reflects the collective magnetism of all the metal ions within the gem. A hand-held spectroscope can help us identify which specific metals are present. An electronic spectrometer is a more sensitive instrument that permits a more detailed analysis of the metallic components, their relative concentrations and even their valence states.
In our lab, a UV-Vis-NIR spectrometer (Ultraviolet, Visible spectrum, and Near-Infrared) helps us identify specifically which metals within a gem are responsible for the color and/or magnetism. When we look at an absorption spectrum represented as a spectrometer graph like the one below, we can see that the peaks on the graph correspond to the black lines and bands seen with a spectroscope. These peaks represent the wavelengths or areas of the light spectrum that are absorbed by specific metal ions. Gems that do not contain paramagnetic metals show no peaks. Manganese and copper are clearly indicated in the graph below for green Paraiba Tourmaline. We have determined that the low concentrations of copper ions (Cu2+) found in Paraiba Tourmalines are not magnetically detectable.
Green Paraiba Tourmaline (Mozambique)
Often there are two or more metallic chromophores mixed within a single gem, but a magnet cannot be used to distinguish between them. For example, Paraiba Tourmalines such as the green gem from Mozambique shown below often contain both copper and manganese impurities. The color of this copper-bearing gem is due to copper ions (Cu2+, blue color) mixing with manganese ions (Mn2+, yellow color) to create green color (blue + yellow = green). This Tourmaline shows a very strong magnetic response. Is the strong magnetism due to a high concentration of paramagnetic copper ions, or is it primarily due to the presence of manganese? Could iron (Fe2+) or even a rare earth metal also contribute to the green color and strong magnetic response of this gem?
There is much to learn through the study of gemstone paramagnetism in relation to spectroscopy, but this is an area of gemological research that has yet to be pursued by gemologists. You can find an online spectrometry database showing absorption graphs for many gems at Dr. George Rossman's Spectroscopy Database for Minerals.
Absorption Spectrum Showing Manganese and Copper in a Green Paraiba Tourmaline