Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral
Floatation Over Water
Magnetic fields are generated around the north and south poles of a magnet.
Periodic Table of Elements
Opposite fields attract (north to south). Like fields repel (north against north, south against south).
It is the Transition metals, and at times Rare Earth metals, that act as chromophores in gems. Transition Metals and Rare Earth metals occupy a central location in the Periodic Table of Elements shown below. There are 38 Transition Elements, but only 8 play a principal role as coloring agents in gemstones (see the Periodic Table below, elements # 22-29 in the top row at the center). Most of these transition metals are paramagnetic within gemstones.
Why Are Gems Magnetic?
Gems are magnetic due to the metals contained with them. When we refer to a gemstone as magnetic, we mean the gem is visibly attracted to a strong magnet. The degree of attraction can be noted as weak, moderate, strong, etc. We can separate and identify gems by observing the various responses that different gems show when the magnetic field of a permanent hand-held magnet is applied to them.
Magnetism arises fundamentally from the spin and orbital motion of electrons. When atoms within a metal contain electrons that are not paired with other electrons, the unpaired electrons are free to align themselves with a magnetic field, resulting in magnetic attraction. Some metals (such as gold, silver and lead) don't have enough unpaired electrons to show magnetic attraction, as is the case with most non-metals and organic substances. However, all substances in nature respond to a magnetic field. They are either attracted or repelled by an external magnetic field. Most responses (attraction or repulsion) are too weak to be directly visible using a common magnet.
Permanent magnets such as household and industrial magnets are manufactured from ferromagnetic materials that are magnetized by a strong electric current. The magnets are then able to generate a strong and persistent magnetic field of their own.
2) Friction, which causes resistance to movement of a gem when the gem is attracted to a magnet, is mostly eliminated. There are a couple of ways to reduce friction (the pedulum method and the floatation method), but we will focus on the simplest and most effective method, which is to float the gem over water.
These ions also act as coloring agents (chromophores). For example, the green color of Emeralds is due to the presence of chromium and/or vanadium ions. The blue color of Indicolite Tourmaline is due to iron ions. Color is the result of light interacting with electrons within a substance. As light rays pass through the crystal structure of a gem, metal ions absorb energy from various portions of the light spectrum, removing specific wavelengths and leaving others that we perceive as color. This selective absorption of wavelengths gives rise to an amazing variety of colors and shades.
Metal objects made from iron or iron compounds are noticeably attracted to a magnet. A familiar example is a paper clip that sticks to an office magnet. The strong attraction shown by the paper clip is an example of Ferromagnetism (ferro refers to iron). The office magnet and paper clip are both composed of ferromagnetic materials, and if we rub a paper clip across the surface of the magnet, the clip becomes magnetized. It retains some of the magnetic field induced by the magnet. The induced magnetic field of the paper clip is strong enough to move a compass needle. This simple experiment makes an interesting demonstration of Ferromagnetism.
There are a number of paramagnetic metals found in nature. In some cases, these metals can exhibit either ferromagnetic or paramagnetic behavior, depending on the state of the metal. For example, when iron exists as a solid metal or metal alloy, as it does in a paper clip or an iron meteorite, it is ferromagnetic. But when iron exists in the form of charged atomic particles called ions that are dispersed in solid solution throughout the chemistry of a gemstone, it is generally paramagnetic. Each metal ion is a single atom containing unpaired electrons, and collectively these metal ions are responsible for the magnetic attraction we find in many gemstones.
Magnetized Paper Clip
Powerful neodymium magnets (neodymium-iron-boron) have only been around since the 1980s. Neodymium is the rare earth element component, while the magnet surface is plated with nickel. Magnetic testing of gemstones as described in these pages is only made possible for two reasons:
Overview of Magnetism in Gemstones
Not All Magnets Are Created Equal
Ferromagnetic particles of iron can be detected with a magnet in some synthetic Diamonds, but synthetic Diamonds are the only transparent gemstones that show Ferromagnetism. The magnetism that we most often encounter in natural and synthetic gemstones is a different kind of magnetism called Paramagnetism. This type of magnetism is due to the presence of metals dissolved within the chemistry of gems. Paramagnetism is a weak type of attraction that can be as much as a million times weaker than Ferromagnetism. Only a temporary magnetic field is involved in Paramagnetism, induced in gems only while the magnetic field of an external magnet is being applied. Paramagnetic gems cannot be permanently magnetized. Unlike our paper clip, they cannot retain a magnetic field after a magnet is removed from the vicinity.
The combination of a powerful magnet and an almost frictionless testing method provides us with a means to detect very slight magnetism in gemstones by movement of the stone toward or away from the magnet. The magnet we use is an N-52 grade neodymium magnet (NIB), which is the strongest grade permanent magnet available today. Weaker grade NIB's and Samarium-Cobalt rare earth magnets are available, but they produce weaker responses that are not equivalent to the results presented throughout this website and on the Magnetic Susceptibility Index.
People are generally surprised to learn that gemstones can be magnetic. This is because most gems show no direct response to the magnets that we commonly use around the house. Alnico magnets (aluminum-nickel-cobalt), such as the horseshoe magnet, are many times weaker than rare earth magnets. Ferrite magnets, such as the ceramic refrigerator magnets shown below, can be even weaker.
Floatation Reveals Weak Magnetism
This 3.3ct. Ruby from Thailand is Paramagnetic
1) Recent advancements have made small and powerful neodymium magnets commercially available and affordable.
© 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.
Rare Earth Metals
Among synthetic gems, additional rare earth elements such as Erbium (Er), Holmium Ho), and Samarium (Sm) are sometimes used to produce color. In the coming pages we'll discover how various metals produce different levels of paramagnetism that result in different degrees of magnetic attraction in gems.
Rare Earth metals occasionally appear in gemstones as paramagnetic coloring agents. There are 30 rare earth elements on the Periodic Table, but only 4 are commonly encountered as coloring agents in natural gemstones. On the Periodic Table, rare-earth elements are found in the bottom two rows labeled Lanthanoids and Actinoids.
A magnetic wand made with an N-52 neodymium magnet can detect iron and manganese in gemstones in concentrations that would normally be undetectable without the aid of sophisticated instrumentation.
Transition metals and rare earth metals exist within the chemistry of gemstones as oxides such as iron oxide, chromium oxide and neodymium oxide. We estimate that an N-52 magnetic wand is able to detect paramagnetic metal oxides in gemstones in concentrations as low as approximately 0.1% by weight for iron oxide, 0.25% for chromium oxide and perhaps somewhat less than 0.1% for neodymium oxide. Most paramagnetic trace metals that might be present in concentrations under 0.1% are not magnetically detectable.
Micro-magnetism of Inclusions: At the microscopic and sub-microscopic level within gems and minerals there often exist tiny inclusions of compounds and foreign minerals embedded during geologic formation of the crystal. Some of these inclusions can exhibit various types of weak to strong micro-magnetism due to the trace amounts of transition metals and rare-earth metals they contain. Collectively, the low level of localized magnetism in these miniscule particles is usually too small to be detected with a hand-held magnet.
Although the collective micro-magnetism of particles and metal oxides within a stone cannot be detected or measured when the metals are paramagnetic and in trace amounts, an ultra-sensitive instrument such as a S.Q.U.I.D magnetometer can measure the ferromagnetism of inclusions containing ferromagnetic particles, primarily iron. Mineral samples are usually pulverized in order to take such measurements. Extremely low levels of ferromagnetism can be recorded quantitatively as electromagnetic units, magnetic moments or mass magnetic susceptibility. Such measurements can be of significance to geologists, mineralogists and geophysicists, but in gemology we are not concerned with them for the purposes of gem identification.
We are primarily interested in paramagnetic metals that are dispersed throughout a gem as ions within metal oxides rather than as metallic inclusions. These metal ions often create gem body color and often exist in concentrations high enough to induce detectable magnetic susceptibility. For our purposes, we measure gem paramagnetism qualitatively in terms of visible magnetic responses to a magnetic wand, and quantitatively as volume magnetic susceptibility measured with a Hoover balance (see page 5).