Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral
An Almandine Garnet Picks Up
Pick Up Response
Aluminum is Paramagnetic
© 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.
Pick-Up Responses: Some magnetic gems will actually be picked up (PU) by N-52 magnets. Most Garnets- Almandine, Spessartine, Pyrope and Andradite species- will show a pick-up response due to their very high iron and/or manganese content. The most common Garnets are red, but many other color varieties of Garnet pick up as well. Pyrope Garnets show the weakest Pick-up responses, and Pyropes may easily slip off the end of the magnet during Direct testing.
If the raft is pushed away in the opposite direction from the magnet, you can designate this as an Inert response (I), which indicates the gem is diamagnetic. If the raft slowly follows the magnet as you pull the magnet away from it, you have a Weak response (W), indicating the presence of some paramagnetic metal. If the raft glides easily toward the magnet, you have a Moderate response (M). Rapid movement indicates a Strong response (S). Deciding which response you are seeing is somewhat subjective, but precise readings are neither possible nor necessary for this method. Moderate responses are usually the most difficult to discern, as they can overlap with either Weak or Strong responses.
On rare occasions, a gem will show no movement in either the push or pull direction. In such cases, the inert gem has just enough paramagnetism to cancel out the weak diamagnetism of the body of the gem. In this website, we use the term "inert" to refer to these rare inert responses, as well as to diamagnetic responses.
Technique: Place the gem raft in the bowl away from the edges. The raft will have a tendency to adhere to the sides of the reservoir due to the surface tension of the water. If the gem is faceted, the table should sit face up on the raft (flat surfaces attract best). Avoid bumping the raft with the magnet or breathing directly on the raft, and keep the bowl away from air currents, or you will see movement unrelated to magnetism. Hold the magnet about a half inch (12 mm) from the gem and note what happens. If the raft moves toward the magnet, maintain the 1/2" testing distance as the gem and raft are pulled across the surface of the water.
A no-pick-up response does not eliminate all Garnets. Gems of the Grossular Garnet species do not contain enough in iron to be picked up by a N-52 magnet, and crystals of the Uvarovite Garnet species do not contain enough Chromium to be picked up when they are over 1ct in weight. Grossular Garnet includes Tsavorite, Mali Garnet, Hessonite, and some lesser known varieties. Uvarovite crystals are not found as faceted gems due to their small size and lack of transparency.
Absence of a Direct response in green Garnet allows us to easily separate Tsavorite Garnet (Grossular) from green Demantoid (Andradite) of identical color. Only Demantoid picks up. Similarly, we can make a quick separation between orange Hessonite Garnet (Grossular) and orange Spessartine of identical color. Only Spessartine picks up. In addition, most Chrome Pyropes and Pastel Pyropes contain so little iron that they show only a Drag response, distinguishing them from all other Pyralspites (see the Garnet section for more information about these 2 unusual Pyrope Garnet varieties).
A Red Garnet Impostor: Another strongly magnetic rare or secondary gemstone is Staurolite. When opaque, this reddish brown mineral is commonly known as the Fairy Stone or Fairy Cross, because the natural twinning of its crystals at 90 degree angles makes a perfect cross. Transparent faceted gems look much like red Garnets, and due to high iron content, small transparent gems under 1 carat pick up with an N-52 magnet (but larger gems only drag).
Staurolite is rarely found as transparent crystals, and such crystals are faceted only for collectors. Staurolite is idiochromatic, and the dark red color, magnetic susceptibility, refractive index and specific gravity all overlap with some low-iron Pyrope Garnet varieties. Under a polariscope, the double refraction of Staurolite can be confused with the anomalous double refraction of red Garnet. Fortunately, Staurolite is so rarely encountered as a transparent gemstone, it is very unlikely to be found impersonating Garnet during routine gem identification.
A slick high-gloss surface such as a magazine cover is recommended for direct testing because it reduces friction and shows drag responses slightly better than most standard surfaces like glass, wood, granite or even Formica. However, any clean smooth surface will work. Gems that show a near-drag response on a standard surface can be re-checked on a high-gloss surface for a more definitive drag response.
When using the direct method, make sure the gem is completely dry. Any water or refractive index fluid caught between the surface of the gem and the magnet can create a seal that results in a false drag or pick-up response.
Drag Responses: A few highly magnetic gems can be dragged by a N-52 magnet on a dry flat surface, but they are not magnetic enough to be picked up (except when very small). There are only six transparent primary gemstones magnetic enough to drag when the direct method is applied. They are:
The Direct method is the first step in magnetic testing of unidentified gemstones. A high-gloss magazine cover provides an ideal testing surface.
Orange Spessartine Garnets
Diagnostic: We will further qualify our use of the term diagnostic. When we say we can use a magnet to separate Garnets from all other gems, we are referring to gemstones of natural origin. The laboratory-grown gems GGG (Gadolinium Gallium Garnet) and HPHT lab-created Diamond are transparent gems that can show a pick-up response. Based solely on magnetic response, red and green GGG might be mistaken for Almandine and Demantoid Garnet respectively. GGG is rarely encountered, as this material has not been manufactured as a diamond simulant for some time. Costly HPHT synthetic Diamonds are only sold as Diamonds, hence colored HPHT synthetic Diamonds are not likely to be mistaken for Garnets.
We must also point out that we are discussing the primary natural gemstones commonly found in the marketplace and that students of gem identification are likely to encounter. There are a few transparent exotic stones known only to collectors of rare gems and minerals that will also be picked up by a magnetic wand, even when above 1ct in weight. The most familiar of these are rare examples of transparent Rhodocrosite and Rhodonite gems (see next page), while others include Dunilite, Eosphorite, Pyroxymangite, Siderite, Tantalite, Triplite, Triphylite, Vivianite and Xenotime.
These rarely-seen gems are faceted from idiochromatic minerals that have high concentrations of manganese, iron or rare earth elements, and several are as magnetic as Garnet. If we relied solely on magnetic response and visual appearance, orange Triplite could be incorrectly identified as Spessartine Garnet (both are colored by manganese as Mn2+), and Triphylite might be incorrectly identified as green Color Change Garnet (both contain iron and manganese).
The Floatation Method
For a quick demonstration of the floatation method, crumple a piece of aluminum foil into a ball, drop it into a bowl of water and hold your magnetic wand in front of it. Aluminum is weakly paramagnetic when it is in pure solid metal form. The absence of friction allows the floating ball of foil to be drawn toward the neodymium magnet.
In a similar way, we can detect the weak magnetic attraction of the paramagnetic metals within gems, although such metals exists as ions rather than as solid metals. We use a gem raft and float the gem on top the raft in a reservoir of water. The reservoir can be created from a bowl or shallow pan filled with tap water and made of plastic, glass, ceramic, tin or aluminum.
Bi-color Indicolite Tourmaline
A Drag response (D) is diagnostic for medium to dark blue Indicolite Tourmaline, as it is the only blue gem of average size that drags. Shown below (right) is a bi-color Tourmaline with blue Indicolite Tourmaline on one end and colorless Tourmaline at the other end. This gem provides an interesting demonstration of the drag response. The gem drags when a magnetic wand is brought to the blue end, but the gem shows no direct response at the colorless end.
Mali Garnet, Peridot, Indicolite, Verdelite, Yellow Tourmaline, pink CZ
Demantoid Garnet, a variety of Andradite, is the only transparent green gemstone of average size that picks up, and this response can be considered diagnostic for Demantoid. Orange Garnets with high Spessartine content (Spessartine Garnet, Orange Malayas and Orange Color Change Garnets) are the only orange transparent primary gemstones of average size that pick up. Note that Spessartine Garnet can also be red, in which case it cannot be distinguished from red Almandine Garnet by magnetic response alone.
The Direct Method
The direct method of testing without floatation simply involves touching the magnet directly to the surface of a gem while the gem rests on a smooth dry flat surface. We can use this method to first check an unidentified gemstone for magnetism. If the gem is highly magnetic, it will drag or pick up. But most magnetic gems will show no direct response to a magnet. Most gems must be tested with the floatation method in order for a magnetic response (paramagnetic or diamagnetic) to become visible.
A cylinder of gem jar foam makes an excellent raft on which to rest your gem. It doesn't matter that the foam soaks up a bit of water- it won't affect results unless it becomes heavily saturated. If you prefer, you can leave the foam in the gem jar and float the entire jar, with the lid removed. For floating larger faceted gems, cabs and jewelry pieces, cut a square of Styrofoam, bubble wrap or cork to use as a larger raft.
Standardized Testing: The key to obtaining magnetic responses that are consistent between users and that fall with the ranges listed on the Magnetic Susceptibility Index is to use a common set of tools and techniques for testing. After experimenting with numerous magnets of different sizes and strengths, we determined that a ½” X ½” N-52 grade neodymium magnet is an optimum size and strength for testing most gemstones. Magnets of other sizes and grades have different pull forces and can show different responses.
The optimum testing distance between the magnet and the gem during floatation is about ½” (12mm). That approximate distance must be maintained as the gem is pulled toward the magnet. We hold the magnet closer to the gem only when we are trying to detect an extremely weak response. Otherwise, holding the magnet closer than 1/2" can result in a false Moderate response from a weakly magnetic gem, or a false Strong response from a moderately magnetic gem. Conversely, holding the magnet further than 1/2" from a gem can result in a diminished response, or no response at all.
Pale examples of any of the above gemstones may require floatation, and show only a weak to moderate response. Two rare varieties of Garnet- Chrome Pyrope and Pastel Pyrope- also show drag responses. Some uncommon secondary gemstones such as Axinite (Ferroaxinite and Manganaxinite species) also show drag responses.
Summary of Magnetic Testing Methods: When testing gems, the Direct Method can be applied first to check for strong magnetism. A pick-up response is generally diagnostic for Garnet when identifying gems that are transparent and of average size. Large Garnet gems and Garnet roughs with irregular surfaces may not pick up, but most types will drag. For transparent gems other than strongly magnetic Garnet, Peridot, and Tourmaline, magnetic testing using the near-frictionless Floatation method is usually necessary to separate one gem type from another in order to narrow the possibilities for identification.
Except in cases when magnetic testing results are diagnostic for a particular gem, additional testing using other gem identification tools is usually required to arrive at a final identification. This is true even for most Garnets when we want to determine the particular species or variety of Garnet.
1) Mali Garnet
3) Blue Indicolite Tourmaline
4) Green Verdelite Tourmaline
5) Yellow Manganous Tourmaline
6) Pink CZ (Cubic Zirconia, man-made)
A Demantoid Garnet Picks Up
Summary of Possible Magnetic Responses
A pick-up response is diagnostic for Garnet, which means: if the gemstone is transparent and of average size (1ct to 4cts), no other test is required to positively identify the gem as Garnet. To determine the species of variety of Garnet, additional testing is usually required.