by Allan Taylor

Created on: July 08, 2011   Last Updated: July 09, 2011

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),

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