Physics

Testing Gems for Magnetism



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The most common magnetic mineral is appropriately named "magnetite" an iron oxide of chemical composition Fe3 O 4. In olden days it was called lodestone because pointy fragments when suspended on a string would line up pointing north and south according to the earth’s magnetic field. Since then we have learnt how to magnetize iron needles and make compasses to determine direction.

Since the 1950s strong magnets have become available made from exotic iron alloys. Alnico magnets are made from iron alloyed with aluminum, nickel and cobalt and are much stronger than pure iron magnets. Stronger still are the rare earth magnets incorporating neodymium which were developed in the 1990s and are now comparatively inexpensive. The iron alloy most used has composition Nd2 Fe14 B or NIB for short. These magnets of any size must be handled with care to avoid accidental bodily harm.

The potential usefulness of magnetism in gem testing using an Alnico magnet was first investigated by B. W. Anderson and results are given in his book "Gem Testing", and repeated in Webster’s "Gems" (1st edition 1962). The ready availability now of the stronger neodymium (Nd) magnets for little cost has renewed interest in this phenomenon.

For gem and mineral testing purposes I have two types of "magnetic wand" which are easy to make. The first is a wand or pencil-like gadget made of ¼ inch plastic rod 5 inches long. One end has cemented a 5 mm dm Nd magnet and the other end has a steel point useful for scratching and hardness testing. This I carry with my small gem testing gear contained in an old rectangular tobacco tin. The wand is strong enough to just pick up a 210 gram can of salmon.

The other magnetic wand is simply a 3 ½ inch steel bolt with a 12 mm dm Nd magnet attracted to the end. This is so strong that I keep it well away from anything and use it for special testing purposes. It will just pick up a 535 gram can of soup.

There are three ways of using these wands to test samples for magnetism. Firstly, tiny mineral samples or small gemstones (less than 2 carat or 0.4 grams) can show a magnetic attraction when resting on a sheet of paper and approached closely with the magnet. They may wobble or be dragged along, or even be picked up if strongly magnetic. The larger wand will pick up a 3 carat red garnet. Try testing your collection of red garnet gemstones. You may be surprised to find some non or weakly magnetic ones which could be ruby (natural or synthetic), red spinel, tourmaline or paste, Obviously the weight of the sample and friction with the paper has a damping effect on the sensitivity of this simple testing method. To overcome this you can try the suspension method and the flotation method.

Useful for larger mineral samples and gemstones is to freely suspend them by a string or thread and approach them closely with the magnet. The sample may move slightly or be pulled by the magnet, or leap onto it. Even red garnets set in jewelry can be detected by this method as garnets are strongly attracted and rubies not  (also, synthetic and most natural rubies fluoresce red under LW UV light whereas garnets don’t).  Alternatively, you can suspend your magnetic wand by a string and observe its reaction when you bring closely a mineral or rough gemstone sample.  You can at least determine whether your sample is inert, or shows weak, moderate or strong magnetism.

Small gemstones or mineral samples can be placed on a slice of cork which is floated on water in a bowl. This flotation method overcomes any friction and is very sensitive for detecting weak magnetism. The method has been pioneered by Kirk Feral who explains all and gives results for most gemstones on his website.

How useful is magnetism? We are still learning of possible applications. Prospectors of gold and minerals will find it useful and so too will gem collectors and gemologists.

Gold prospectors who use a gold pan to pan off a heavy residue containing gold have always utilized a magnet to remove the black sand which is composed predominately of tiny grains of magnetite, but not always. Even a weak iron magnet enclosed in paper will do this.

Mining companies have a more in-depth interest in heavy residues obtained from pan-concentrate surveys that may be done over a large area of an Exploration License. Often hundreds of samples are obtained and examined with a binocular microscope to identify the minerals present, which is not easy. Gold particles are usually readily identified (unless coated with amalgam etc.), but besides magnetite there is a host of heavy minerals that could be present, such as garnet, zircon, tourmaline, chromite, pyroxenes, amphiboles, epidote, xenotime, tantalite, scheelite, corundum, diamond and so on, all of which have different magnetic susceptibilities. The skilled geologist tries to identify these tiny grains by observing first their crystal habit (e.g. magnetite is octahedral, chromite and corundum form hexagonal tablets, tourmaline is prismatic and strongly dichroic etc.). Now with a Nd magnet wand one can usefully remove successive "magnetic layers" of heavy minerals, which is a help in identification. In a particular watershed the results tell you what rock types are being eroded upstream and the possible existence of ore deposits.

The most useful application for gemologists or gem collectors is the ready identification of most garnets. Garnets that contain appreciable iron and manganese are strongly magnetic, which includes the red garnets pyrope-almandine, rhodolite and the orange garnet spessartite and the green garnet demantoid, so that small gemstones of them may be picked up by a magnet. Other garnets that show lesser magnetism (still moderate to strong and may wobble or drag) are tsavorite, hessonite, grossular and Mali garnet, the reaction being dependent iron, manganese, chromium and vanadium content.

Magnetism is caused by the presence of transition metal ions in the structure of the mineral and these ions cause the color due to selective absorption of light transmitted. Colorless minerals have no such impurities and are not magnetic (topaz), or gems whose color is due to lattice defects (quartz, diamond) with no metal ions present also are nonmagnetic. Iron is the most common metal ion (ferrous and ferric) causing coloring in gems and minerals giving yellow, greens, blue and black, so peridot, tourmalines, chrysoberyl, iolite, green jades show moderate to strong magnetism. The next most common colorant is manganese, which gives a pink color. Both pink rhodonite and rhodochrosite gemstones are strongly magnetic, as are hematite (grey) and malachite (green).

The gem collector can have a pleasant and educational time testing all his gems and become familiar with the phenomenon of magnetism by comparing their responses. For sure he will find new useful applications.







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ARTICLE SOURCES AND CITATIONS
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Lodestone
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Alnico
  • InfoBoxCallToAction ActionArrowhttp://en.wikipedia.org/wiki/Rare_earth_magnet
  • InfoBoxCallToAction ActionArrowhttp://gemstonemagnetism.com