Magnetism in Gemstones 
                       An Effective Tool and Method for Gem Identification 
                                                                                                 © Kirk Feral

Translucent Rhodocrosite
Translucent Rhodonite
OpaqueRed Jasper Cab
Iron Meteorite
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Both shapes of Chrysoprase shown here, faceted and cabochon, show a strong magnetic response when floated.
A Large Garnet (11.5ct) Drags
A melee size Peridot (0.25ct) Picks Up
Opaque Black Tourmaline
Gem Size: When using the floatation method, gems of 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 gems that are weakly magnetic, as they must overcome the slight friction of the gem raft moving across water in order to show a response (see photo below left).
© 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.
Black Gems. Opaque black stones such as black Tourmaline (Shorl), black Garnet (Melanite), black Star Diopside, Obsidian and black aggregate stones are very magnetic because of high iron content, and will often be picked up or dragged by a N-52 magnet. Black Star Diopside will be picked up even by a weak refrigerator magnet. Opaque Black Star Sapphire can vary from drag responses to weak responses.. 
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 and manganese 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.
Serpentine Aggregate
Tiger Iron
Transparent Black Spinel
Natural Hematite Crystal
Opaque Rohdonite
In summary, the direct pick-up method is diagnostic for Garnet when the primary gems and roughs are transparent and of average size. Large Garnet roughs with irregular surfaces may not pick up, but they will usually drag. For gems other than Garnet, magnetic testing using near-frictionless floatation on water is usually necessary to separate one gem type from another and narrow the possibilities. Further testing using other gem identification tools is usually required to arrive at a final identification. This is true even for most Garnets if we want to determine the particular species or variety of Garnet.
Hematite. Most published information about natural Hematite tells us that it is non-magnetic or weakly magnetic, but this is outdated information based on direct testing with common household magnets. Natural Hematite (iron oxide) will be picked up by an N-52 neodymium magnet. However, Hematite does in most instances have a considerably lower magnetic susceptibility than its man-made counterpart "Hematine", and Hematite picks up with a much weaker force than does "Hematine".

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 will not be picked up by a relatively weak horseshoe magnet or a refrigerator magnet, but "Hematine" will jump to these magnets.  
Hematite Inclusions in Topaz
Hematine (man-made)
Factors that Affect Magnetic Responses

Gem Shape and Cut: Whether you 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, you’ll find that variations in gem shape and cut don't significantly alter the responses you see with a magnetic wand.
Gem Weight: This is the most significant factor that affects responses. Weight is irrelevant when you use the floatation method, but pick-up and drag responses are weight-dependant. Large Garnet gems may be too heavy to pick up, but they will drag. Approximate cut-off points between pick-up and drag responses for some 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 with heavy iron content such as native iron Meteorites, Tiger Iron, and "Hematine" (trade name for imitation Hematite used in jewelry), are intensely ferromagnetic and jump to an N-52 magnet. These gems and minerals can be magnetized.
Inclusions of nautral Hematite within other gemstones such as Quartz or Topaz show no attraction to a magnet of any strength, partly because oxidation states have altered the magnetism.
Transparent and translucent black gems like Obsidian can be strongly magnetic, but may not pick up like opaque stones. Transparent black Spinel and transparent Obsidian show a weak to moderate response due to low iron content.
Completely transparent Rhodocrosite and Rhodonite gems are rarely seen and cannot be considered as primary gemstones, but gems of average size are picked up by a magnet as easily as any Garnet. The coral red to cherry red colors are visually distinct from colors found in typical red Garnet gems. Red Garnets generally have darker tones and greater transparency. However, if a magnet were the only identification tool used, transparent Rhodocrosite or Rhodonite could conceivably be mistaken for some Malaya Garnets or Pastel Pyrope Garnets. The transparent Rhodocrosite shown below (left) is the most magnetic natural transparent gemstone known to this researcher, surpassing Spessartine Garnet in measured susceptibility. The transparent Rhodonite shown below (right) is as magnetic as some Spessartines.
Translucent and Opaque Gems That Pick Up

Rhodocrosite and Rhodonite are bright pink to red, translucent to opaque stones. Opaque stones are considered primary gemstones because they 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 show a strong attraction when floated. Rhodocrosite cannot be distinguished from Rhodonite by magnetic response.
Opaque Rhodocrosite
Both of these rose-colored gem species are idochromatic and owe their color to high concentrations of manganese in their chemical composition. Faceted 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 with high concentrations of iron impurities or large inclusions containing iron can exhibit ferromagnetic, ferrimagnetic and antiferromagnetic responses. These are types of magnetic responses far stronger than paramagnetic responses. Some ornamental mineral cabs, and cabs fashioned 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, Serpentine, Nephrite, 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 concentration of iron is lower.
Some black gems are magnetically inert. Black Carbonado Diamond, which is made of non-magnetic carbon, is an example. Black organic gems such as black Coral, black Pearl and Jet do not contain paramagnetic metals and show no magnetic responses. Black Onyx, colored by microscopic inclusions of iron oxide compounds, also shows no magnetic attraction. Several reasons might be responsible for the lack of attraction in Onyx: 1) the iron oxides may be in valence states that cannot be magnetically detected; 2) iron may be in concentrations too low to generate a magnetic response; and 3) much of the iron might be tied up in charge transfer processes that cause black color. 
Black Onyx
Transparent Rhodocrosite 5.0ct.
Transparent Rhodonite 4.61ct
When a magnet is held in front of a large 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 Beryl pictured below (right) shows a moderate magnetic response when the floatation method is used even though its actual susceptibility is very weak at only 22 X 10(-6) SI.
Testing a Small Gem
Testing a Large Gem