Magnetism in Gemstones
An Effective Tool and Method for Gem Identification
Opaque Red Jasper Cab
Iron Meteorite (Picks Up)
Both shapes of Chrysoprase shown here, faceted and domed cabochon, show strong magnetic responses when floated.
A Large Garnet (11.5ct) Drags
A Very Small Peridot (0.25ct) Picks Up
Opaque Black Tourmaline (Drags)
Gem Size: When using the floatation method, gems of very small size may respond less obviously than larger ones because of the small surface area subject to the magnetic field. This is most noticeable in tiny (melee) gems that are weakly magnetic, as they must overcome inertia and slight friction of the gem raft moving across water in order to show a response (see photo below).
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Black Gems: Opaque black stones such as black Tourmaline (Shorl), black Garnet (Melanite), black Star Diopside, black Obsidian, Psilomelane and black aggregate stones are very magnetic because of high iron and/or manganese content, and will often be picked up or dragged by an N-52 magnet. Black Star Diopside will be picked up even by a weak refrigerator magnet. Opaque Black Star Sapphires are only weakly to moderately magnetic.
The five highly magnetic transparent primary gems other than Garnet that show a drag response can also show a Pick-up response when those gems are exceptionally small and light. Approximate cut-off points between Drag and Pick-up responses for these gems are: Peridot and pink CZ may pick up under 0.5ct.; Indicolite, Verdelite green Tourmaline and manganous yellow Tourmaline may pick up under .35ct.
When very small, Peridot might be confused with Demantoid Garnet, as both green stones pick up. But only Demantoid jumps to the magnet.
Tiger Iron (Picks Up)
Transparent Black Obsidian
Natural Black Hematite
Hexagonal Crystal (Drags)
Hematite: Hematite is a natural iron oxide mineral that forms hexagonal crystals. Most of the published information about natural Hematite informs us that it is non-magnetic or weakly magnetic, but such information is based on direct testing with common household magnets. Natural Hematite is very magnetic and will be picked up by a neodymium magnet. However, Hematite does in most instances have a considerably lower magnetic susceptibility than its man-made counterpart "Hemetine", which is an iron oxide compound manufactured with barium and strontium additives. Natural Hematite is a paramagnetic mineral, while man-made "Hemetine" is a ferromagnetic material. Hematite picks up with a much weaker force than does "Hemetine".
with Hematite Inclusions
Tumbled Pebble (Picks Up)
Factors that Affect Magnetic Responses
Gem Shape and Cut: Whether we are using the Floatation method or Direct method, magnetic response is somewhat affected by a gem’s shape and cut. The larger the flat surface area of the gem when held parallel to the magnet, the better the magnet is able to pull the gem. Cabochons with dome tops don’t pull quite as easily as faceted gems, and pear cuts don’t pull quite as well as square cuts with larger facet tables and surface areas. But all exposed gem surfaces are subject to the magnetic field of a magnet. In most cases, we find that variations in gem shape and cut don't significantly alter the responses we see with a magnetic wand.
Gem Weight: Gem weight is irrelevant when we use the floatation method, but Pick-up and Drag responses are weight-dependent. This is by far the most significant factor that affects Direct magnetic responses. Larger Garnet gems and faceting rough may be too heavy to pick up, but most will drag. Approximate cut-off points between Pick-up and Drag responses for Garnets are: Spessartine over 4-5ct, Almandine and Andradite over 2-3ct, Pyrope, Rhodolite and Malaya over 1-2ct, Chrome Pyrope over 1ct, Mali Garnet over 0.5ct.
Metallic Cabs and Minerals: Cabs and mineral specimens containing solid iron, iron-nickel alloys or iron compounds can be intensely ferromagnetic and jump to an N-52 magnet. Examples include Meteorites, Tiger Iron, and man-made "Hemetine", an imitation of Hematite used in jewelry. These gems and minerals can retain a weak magnetic field of their own after being exposed to a magnet.
When cut in thin section, Hematite appears red rather than black or gray. Transparent natural gemstones such as Hematoidal Topaz, "Strawberry" Quartz and Oligoclase "Sunstone" (a.k.a. "Confetti Stone"), all pictured below, contain natural red inclusions of Hematite. The hematoidal inclusions can be extremely thin. These gems show no attraction to an N-52 grade neodymium magnet, probably because the amount of iron is so small.
Translucent black gems like Obsidian can be strongly magnetic, but may not pick up like opaque stones. Transparent bluish-black Spinel and transparent brownish-black Obsidian show weak to Moderate responses due to relatively low iron.
Completely transparent faceted Rhodochrosite and Rhodonite gems are rare and expensive, but transparent gems of average size will show a Pick-up response as easily as any Garnet. Body color is often different from colors found in typical red Garnet gems. However, transparent red Rhodochrosite from South Africa (below left) can closely resemble red Garnet, and orange Rhodochrosite can resemble Spesssartine. The rare transparent pink Rhodochrosite from China shown below (center) is the most magnetic natural transparent gemstone we have tested, surpassing Spessartine Garnet in measured susceptibility. If a magnet were the only identification tool used, transparent Rhodochrosite and Rhodonite could be mistaken for pink or red Malaya Garnet. The rare transparent Rhodonite from Brazil shown below (right) is as magnetic as red Spessartine Garnet.
Translucent and Opaque Gems That Pick Up
Rhodochrosite and Rhodonite are bright pink to red, mostly translucent to opaque stones that can show a Pik-up response. Opaque stones are commonly used as ornamental lapidary materials in the form of opaque slabs, cabs and beads. Large cabs can be too heavy to pick up with a magnet, but they show a strong attraction when floated. Rhodochrosite cannot be distinguished from Rhodonite by magnetic response.
Both of these red gem species are idiochromatic and owe their color to high concentrations of manganese within their chemical compositions. Faceted translucent Rhodochrosite and Rhodonite gems are rare, and they characteristically have a foggy translucent appearance. They are highly magnetic, and gems of average size can be picked up by an N-52 magnet.
Opaque Cabs and Aggregates: Opaque stones that have high concentrations of iron impurities or large inclusions containing iron can exhibit ferromagnetic, ferrimagnetic and antiferromagnetic responses. These types of magnetic responses are all far stronger than paramagnetic responses. Some ornamental mineral cabs, and some cabs fashioned as gemstones from rocks (aggregates of minerals), will stick to a magnet. Pick-up and Drag responses can be seen with cabs containing various minerals such as red Jasper, green Serpentine, black Nephrite Jade, and red Rhyolite. However, magnetic responses may not help with gem identification. Other cabs and rocks with similar colors and the same minerals may not pick-up or drag because the concentrations of iron and manganese are lower.
Some black gems are magnetically inert (diamagnetic). Black Carbonado Diamond, which is composed of non-magnetic carbon, is an example. Other examples include black organic gems such as black Coral, black Pearl and Jet, none of which contain paramagnetic metals. Natural black Onyx is a Chalcedony variety colored by microscopic inclusions of iron oxide compounds, and this gem also shows no magnetic attraction. There are several possible reasons why Onyx is not magnetic: 1) dispersed ions of iron may be in concentrations too low to generate a magnetic attraction, as is the case with orange Carnelian Chalcedony; 2) black color may be due primarily to intervalence charge transfer processes that do not produce magnetic susceptibility; 3) the Onyx may be dyed. Most black Onyx on the market today is dyed.
Black Onyx (Inert)
China, (Picks Up)
Australia, (Picks Up)
When a magnet is held in front of a gem that has a large flat surface area at its table facet, the gem may show magnetic attraction that is disproportionate to its actual magnetic susceptibility. The large green Beryl with a very large table facet pictured below shows a moderate magnetic response when the floatation method is used. However, its actual measured susceptibility is weak at only 22 X 10(-6) SI.
Testing a Small Gem with a Small Surface Area
Testing a Large Gem with a Large Table Facet
"Iron Rose" (Drags)
South Africa, (Picks Up)
with Hematite Inclusions
To clearly separate natural Hematite from man-made imitations, the direct method using a household magnet instead of a neodymium magnet is actually the preferred test. Most natural Hematite, including native varieties such as hexagonal crystals, "Iron Rose" Hematite and glittery "Specular" Hematite, will not be picked up by a relatively weak horseshoe magnet or a refrigerator magnet. In contrast, man-made "Hemetine" beads and cabs will often jump to these weaker magnets.
with Hematite Inclusions
We must be careful not to identify green Tourmaline or Peridot gems as Demantoid Garnets simply based on the Pick-up responses of these gems when they are very small (less than a third of a carat).
Transparent Bluish Black Spinel
Banded Onyx Cabachon (Inert)
Opaque Black Obsidian (Drags)
The iron in the red Hematite inclusions in the gems above are paramagnetic, but the quantity of iron oxide that composes the extremely thin red platelets is so small that it cannot be detected with a magnetic wand.
The crown of a gem with its flat table facet will pick up or drag more readily than the tapered end of the pavilion side with its small facets, and an un-faceted rough with irregular surfaces will pick up or drag significantly less readily than a cut gem with a smooth flat surface. However, when floatation is used, the pavilion side of cut stones and the irregular surfaces of rough stones respond to a magnet nearly to the same degree as the crown side of cut stones.
Pavilion of a Faceted Peridot
Such variations in magnetic response between gems of different shapes, sizes, cuts and surface areas are not significant enough to jeapordize the overall reliability of magnetic testing, as we use the observed responses only as approximate indicators of magnetic susceptibility. Differences between Moderate and Weak responses, or Moderate and Strong responses, may be occasionally obscured during Floataion, as well as differences between Drag and Pick-up responses during Direct testing, but these variations are rarely great enough to hinder our ability to make clear distinctions between Diamagnetic, Weak, Strong and Direct responses.