Garnets are a diverse group of silicate minerals renowned for their beautiful colors and substantial hardness, frequently used as gemstones and industrial abrasives. This mineral group shares a common crystal structure but exhibits a wide range of chemical compositions, giving rise to distinct species. The question of whether garnets are magnetic is common, and the answer is not a simple yes or no, but rather a spectrum of responses. The magnetic behavior of any given garnet specimen is linked to its specific internal chemistry, which dictates the strength of its reaction to a magnetic field.
The Direct Answer
The majority of garnets are not ferromagnetic, meaning they will not stick to a common refrigerator magnet like iron or steel. However, many garnet species are paramagnetic, exhibiting a measurable attraction to a strong magnetic field. This distinction is significant because garnets are unique among transparent gemstones for showing a reliable, observable magnetic attraction. The variability in magnetic response ranges from essentially inert (diamagnetic) to highly magnetic, capable of being picked up by a powerful magnet.
The least magnetic garnets typically belong to the Ugrandite series, specifically pure Grossular and Pyrope. Conversely, the most magnetic species are found in the Pyralspite series, including Almandine and Spessartine, as well as Andradite from the Ugrandite group. Spessartine garnet, colored primarily by manganese, often exhibits the highest magnetic susceptibility value among all gem garnets.
This difference in magnetic response is routinely used by gemologists for identification purposes. For example, Spessartine and Almandine are often strongly magnetic enough to be physically picked up by a powerful magnet. In contrast, Grossular varieties like Tsavorite are much less magnetic, sometimes only showing a weak drag or no direct attraction at all. This spectrum of magnetic properties is an effective tool for distinguishing between the various species within the garnet group.
Chemical Basis for Magnetic Properties
The magnetic behavior in garnets is fundamentally tied to the presence of specific elements known as transition metal ions within their crystal structure. These ions include iron (\(\text{Fe}^{2+}\) and \(\text{Fe}^{3+}\)), manganese (\(\text{Mn}^{2+}\)), and chromium (\(\text{Cr}^{3+}\)). These metals are paramagnetic, meaning they possess unpaired electrons that align temporarily with an external magnetic field, causing the material to be drawn toward the magnet.
The concentration of these paramagnetic ions directly determines the strength of the garnet’s magnetic attraction. Garnet species whose color is caused by an intrinsic component of their chemical formula (idiochromatic) typically have higher concentrations of these magnetic metals. For instance, Almandine is rich in ferrous iron (\(\text{Fe}^{2+}\)), while Spessartine is characterized by manganese (\(\text{Mn}^{2+}\)). Magnetic susceptibility increases proportionally with the amount of iron or manganese incorporated into the composition.
Garnets with minimal to no transition metals, such as pure Pyrope and Grossular, are allochromatic, meaning any color they exhibit is due to trace impurities, and their magnetic response is nearly inert. The crystal structure accommodates these metal ions in specific lattice sites. The exact position and valence state of the iron or manganese ion influences the ultimate strength of the attraction, making the specific chemical formula a precise predictor of the magnetic response.
Field Testing for Magnetism
Testing a garnet’s magnetic response in a practical setting requires a strong rare-earth magnet, typically a Neodymium grade N52 magnet. This type of magnet is necessary because the paramagnetism in garnets is often too subtle to be detected by weaker, conventional magnets. The standard procedure involves using a magnetic wand fashioned by attaching the Neodymium magnet to a small handle.
For loose gemstones, a common method is the floatation test, where the garnet is placed on a small raft, like a piece of foam, floating on water. The magnetic wand is then brought close to the stone, and the degree of attraction or movement is observed. A strong magnetic garnet will move quickly toward the magnet or even be picked up directly if the magnet is powerful enough and the stone is small.
A less magnetic garnet may only show a slight drag response when the magnet is moved near it, or it may remain completely inert, indicating a diamagnetic response. For mounted jewelry, the magnet can be brought close to the stone itself, though the metal setting may interfere with the results. This magnetic testing is a rapid, non-destructive method that allows gemologists to quickly distinguish genuine garnets from non-magnetic simulants or to separate one garnet species from another.