Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
© Kirk Feral
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 (P/U) by N-52 magnets. Most gems of the Garnet group 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.
If you see no movement, or if the raft is pushed away in the opposite direction from the magnet, you can designate this as a Diamagnetic or Inert response (I). 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. Moderate responses are usually the most difficult to discern, as they can overlap with either Weak or Strong responses.
Some Inert responses show no movement in either the push or pull direction. In these cases, the inert gems have just enough paramagnetism to cancel out the weak diamagnetism that is always present in gems. In most cases, we use the term "Inert" to refer to "Diamagnetic" responses.
Technique: Place the gem raft in the bowl away from the edges. The raft will have a tendency to stick to the sides of the reservoir because of the surface tension of the water. If the gem is faceted, the table should face up (flat surfaces attract best). Avoid breathing directly on the gem raft or touching it with the magnet, or you will get a false response. Hold the magnet about a half inch (12 mm) from the gem and note what happens. Don't hold the magnet closer to the gem unless you are trying to detect an extremely weak response. Holding the magnet closer than ½” from a gem can cause a false Moderate response from a Weak gem, or a false Strong response from a Moderate gem. Holding the magnet further than 1/2" from a gem can result in a falsely diminished response or no response.
A no-pick-up response does not eliminate all Garnets. One Garnet species is not saturated enough in iron or manganese to be picked up by a N-52 magnet. This is Grossular Garnet, which includes Tsavorite, Mali, Hessonite, and some lesser known varieties. A no-pick-up response in green Garnet allows us to easily separate Tsavorite Garnet (Grossular) from Chrome Demantoid of identical color. Similarly, we can make a quick separation between orange Hessonite Garnet (Grossular) and orange Spessartine of identical color. In addition, most Chrome Pyropes and Pastel Pyropes are so low in iron they only show a Drag response (see the Garnet Magnetism section for more information about these 2 unusual Pyrope varieties).
The Red Garnet Impostor: Another strongly magnetic 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 rough is rarely found as transparent crystals, and it is 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 easily 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.
When using the direct method, make sure the gem is completely dry. A drop of water between the surface of the gem and the magnet can create a water 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. They are: Mali Garnet, Peridot, blue Indicolite Tourmaline, green Verdelite Tourmaline, brownish yellow Tourmaline, and pink CZ. (Note: Pale examples of any of these gemstones may require floatation, and show only a weak to moderate response). Two uncommon varieties of Garnet- Chrome Pyrope and Pastel Pyrope- also show drag responses. Regarding the uncommon secondary gemstones, a few such as Axinite also show drag responses.
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
We will further qualify the term diagnostic. When we say we can use a magnet to separate Garnet from all other gems, we are referring to gemstones of natural origin. Three laboratory-grown gems, GGG (Gadolinium Gallium Garnet) and synthetic HPHT Diamond, are transparent gems that can show a pick-up response. Red and green GGG might be mistaken for Almandine and Demantoid Garnet based solely on magnetic response. But GGG is hard to find, as this material has not been manufactured as a diamond simulant for some time.
We must also point out that we are discussing the primary natural gemstones commonly found in the marketplace. There are a few transparent exotic gemstones 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 (see next page), while others include Eosphorite, Siderite, Tantalite, Triplite, Triphylite, Vivianite and Xenotime. These rarely-seen gems are faceted from idiochromatic minerals that have high concentrations of iron, manganese 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, and Triphylite might be incorrectly identified as a Color Change Garnet.
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 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 in gems. 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.
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 saturated. If you prefer, you can leave the foam in the gem jar and float the entire jar, with the lid removed. For larger faceted gems, cabs and jewelry pieces, cut a square of Styrofoam or bubble wrap to use as a raft.
Bi-color Indicolite Tourmaline
A Drag response (D) is diagnostic for medium to dark blue Indicolite, 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 achroite Tourmaline at the other end. This gem provides an interesting demonstration of the drag response. The gem drags when a magnetic wand is brought near the blue end, while the gem shows no response at the colorless end.
Mali Garnet, Peridot, Indicolite, Verdelite, Yellow Tourmaline, pink CZ
This response is diagnostic for Garnet, which means: IF THE GEMSTONE IS TRANSPARENT AND OF AVERAGE SIZE (1ct - 4ct), NO OTHER TEST IS REQUIRED TO POSITIVELY IDENTIFY THE GEMSTONE AS GARNET. To determine the species or variety of Garnet, further testing is usually required.
Demantoid Garnet is the only transparent green gemstone of average size that picks up, and this response is diagnostic for Demantoid. Orange Garnets with high Spessartine content (Spessartine Garnet, Orange Malayas and Orange Color Change Garnets) are the only orange transparent gemstones of average size that pick up. Note that Spessartine Garnet can also be red, in which case it cannot easily be distinguished from Almandine Garnet or Pyrope Garnet with a magnetic wand.
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 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 not show a visible direct response to a magnet, and will likely need to be tested with the floatation method.