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).
Why Are Gems Magnetic?
In this website, we use the general term "magnetic" to refer to any gem that shows visible attraction to a magnet. Magnetic gems are attracted to a magnet because of the metals they contain. The degree of attraction can be noted as weak, moderate, strong, drags or picks up. Many gems are not "magnetic", meaning they are not attracted to a magnet, and these show an inert or diamagnetic response. We can separate one type of gem from another, and sometimes identify a gem, simply by observing which response a gem shows when the magnetic field of an N-52 grade neodymium magnet is applied to it.
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 only use the simplest and most effective method, which is to float the gem over water. See page 2 of How to Use a Magnet for details.
These metal ions within oxides can 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 then perceive as color. This selective absorption of wavelengths gives rise to a large 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 then strong enough to move a compass needle. This simple experiment is 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 pure 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. These metal ions are chemically bonded to oxygen atoms, existing as metal oxides within gems.
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.
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 transparent 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 the 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 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 as movement of the gem toward (or away) from the magnet. The magnet we use is an N-52 grade neodymium magnet (neodymium-iron-boron magnet, or NIB), which is the strongest grade permanent magnet available today. Weaker grade neodymium magnets such as N-42 NIB's, and other rare-earth magnets such as Samarium-Cobalt magnets, are available, but these are less effective tools that generate weaker responses than those presented in 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 (right), can be even weaker.
Floatation Reveals Weak Magnetism
This 3.3ct. Ruby from Thailand is Paramagnetic
Magnetic testing of gemstones as described in this website is only made possible for two reasons:
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, other Rare Earth elements such as erbium (Er), holmium (Ho), and europium (Eu) are sometimes used to produce pink colors. 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 17 Rare Earth elements on the Periodic Table (next to the bottom row), but only 3 act as coloring agents in natural gemstones: neodymium, praseodymium and cerium. Uranium is a radioactive paramagnetic metal found as a coloring agent only in natural Zircon gems (see the bottom row of the Periodic Table).
A magnetic wand made with an N-52 neodymium magnet can detect these metals in gemstones in concentrations that would normally be undetectable without the aid of more sophisticated instrumentation.
How Sensitive is Our Magnetic Wand?
Our magnetic wand is quite sensitive, but the exact level of detectability for different metals is not known. Transition metals, Rare Earth metals and uranium exist within the chemistry of gemstones as oxides such as iron oxide, manganese oxide, chromium oxide and neodymium oxide. Based on the known concentrations of these metal oxides within different gem types, and also the known magnetic susceptibilities of these metal oxides relative to each other, we estimate that an N-52 magnetic wand is able to detect these paramagnetic metal oxides by weight within gemstones in concentrations as low as approximately:
0.1% iron oxide (FeO)
0.13% manganese oxide (MnO)
0.4% chromium oxide (Cr2O3) / vanadium oxide (V2O3)
0.1% cobalt oxide (Co3O4)
0.07% neodymium oxide (Nd2O3) / praseodymium oxide (Pr2O3)
Most paramagnetic metals that are present in natural gems in trace amounts (under 0.1%) are not detectable with a 1/2" X 1/2" N-52 neodymium magnet. More exact data on the magnetically detectable concentrations of metal oxides within gemstones and their measured magnetic susceptibilities using neodymium magnets could be discovered through research that compares magnetic susceptibility data for specific gem samples with chemical composition data on the same samples obtained with electron microprobe or LA-ICP-MS methods. This line of inquiry has not yet been been pursued by gemologists.
Micro-ferromagnetism: 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 (paramagnetism, ferromagnetism and other types of 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 neodymium magnet.
An ultra-sensitive instrument called a S.Q.U.I.D magnetometer is able to measure micro-magnetism within micro-inclusions, but only when the magnetism is ferromagnetic, produced primarily by particles of iron compounds. Gem and mineral samples must be 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 are mostly of significance to geologists, mineralogists and geophysicists, but in gemology we are not concerned with them for the purposes of gem identification.
It is the Transition metals, and at times Rare Earth metals and even uranium, that act as chromophores in gems. Transition Metals, Rare Earth metals and uranium occupy a central location in the Periodic Table of Elements. 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). Most of these transition metals are paramagnetic within gemstones.
When using magnetic testing to help us identify gems, we are primarily concerned with detecting paramagnetic metals that are dispersed throughout a gem as ions within metal oxides rather than as microscopic metallic inclusions embedded in gems. 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 magnetic susceptibility balance (see page 5).