Several semi-precious stones exhibit magnetic properties, primarily due to the presence of iron, manganese, or chromium within their mineral composition. While some, like lodestone, are naturally magnetic (ferromagnetic), most display paramagnetism, meaning they are attracted to a strong magnet and can show a "pick-up" or "drag" response.
Understanding Magnetism in Gemstones
The magnetic response in gemstones is typically a result of their chemical makeup, particularly the presence of certain transition metals.
- Ferromagnetism: This is the strongest type of magnetism, where a material can become permanently magnetized. Lodestone (a variety of magnetite) is a rare example of a naturally ferromagnetic gemstone.
- Paramagnetism: Most magnetic semi-precious stones are paramagnetic. This means they are weakly attracted to a magnet but do not retain magnetism once the magnet is removed. The attraction often becomes noticeable only with a strong rare-earth magnet (e.g., N52 grade neodymium magnet). The degree of attraction depends on the concentration of magnetic elements like iron (Fe), manganese (Mn), and chromium (Cr).
- Diamagnetism: Many common gemstones are diamagnetic, meaning they are very weakly repelled by a magnetic field. This effect is usually imperceptible without specialized equipment.
Naturally Magnetic Semi-Precious Stones
These stones are known for their distinct magnetic properties, often strong enough to be easily observed.
Lodestone (Magnetite)
Lodestone is a naturally magnetized form of the mineral magnetite (iron oxide, Fe₃O₄). It is unique among semi-precious stones for its ability to attract and hold small metallic objects without being exposed to an external magnetic field. Historically, it was used in early compasses. Lodestone typically appears as a black or dark gray, opaque stone with a metallic to dull luster.
- Magnetic Response: Strongest, naturally magnetized (ferromagnetic). Can pick up small paper clips or interact with a compass.
- Appearance: Opaque, metallic black to dark gray.
Hematite
Hematite (iron oxide, Fe₂O₃) is another semi-precious stone known for its significant iron content, making it strongly paramagnetic. While not naturally magnetized like lodestone, a polished hematite stone will often stick to a strong rare-earth magnet and show a distinct drag response. Its metallic luster and dark color make it a popular choice for jewelry and ornamental carvings.
- Magnetic Response: Strong paramagnetism; sticks well to strong magnets, showing "pick-up" and "drag."
- Appearance: Metallic gray to black, often with a reddish streak.
Semi-Precious Stones Exhibiting Noticeable Magnetic Responses
Beyond lodestone and hematite, several other semi-precious stones, particularly those rich in iron or other magnetic elements, will exhibit a noticeable attraction to a strong magnet. Ornamental mineral cabs and cabs fashioned from rocks (aggregates of minerals) are often observed to stick to a magnet or display distinct pick-up and drag responses.
Jasper
Various types of jasper, a form of chalcedony, can show magnetic properties. This is due to inclusions of iron oxides within the stone. Red Jasper, in particular, often contains significant iron, contributing to its vibrant color and making it noticeably attracted to a strong magnet.
- Magnetic Response: Moderate to strong paramagnetism, especially iron-rich varieties.
- Appearance: Opaque, wide range of colors; red, brown, green, yellow.
Serpentine
Green serpentine varieties, often used for ornamental purposes, can contain iron and chromium, which impart paramagnetic properties. Some green Serpentine cabs will stick to a magnet, showing a clear magnetic interaction.
- Magnetic Response: Moderate paramagnetism.
- Appearance: Opaque to translucent, typically green, yellow-green, or brownish-green.
Nephrite Jade
Some varieties of nephrite jade, a tough and durable semi-precious stone, contain enough iron to be attracted to a magnet. Black Nephrite Jade is one such example that can exhibit a noticeable magnetic pull.
- Magnetic Response: Mild to moderate paramagnetism due to iron content.
- Appearance: Opaque, typically green, white, gray, or black.
Rhyolite
Rhyolite is a volcanic rock that can be fashioned into attractive cabs. Its magnetic properties come from the inclusion of magnetic minerals like magnetite or hematite within its aggregate structure. Red Rhyolite, for instance, can show a distinct magnetic response.
- Magnetic Response: Mild to moderate paramagnetism, depending on magnetic mineral content.
- Appearance: Fine-grained, typically pink, gray, or red, often with flow patterns.
Garnets
Certain garnet varieties, particularly those rich in iron and manganese, are strongly paramagnetic. Almandine and pyrope garnets are notable examples that can be easily picked up by a strong neodymium magnet and show a clear "drag" response, making them popular for magnet testing by gemologists.
- Magnetic Response: Strong paramagnetism.
- Appearance: Translucent to opaque, typically deep red, reddish-brown.
Tiger's Eye
Tiger's Eye, a chatoyant variety of quartz, owes its characteristic golden-brown bands to fibrous inclusions of crocidolite, an iron-rich amphibole. These iron-bearing fibers give Tiger's Eye a mild but discernible paramagnetic response to a strong magnet.
- Magnetic Response: Mild paramagnetism.
- Appearance: Opaque, golden-brown with silky luster.
How to Test for Magnetism in Gemstones
Testing for magnetism in gemstones is a straightforward process, often used by gemologists to help identify stones or assess their composition.
- Obtain a Strong Magnet: Use a powerful rare-earth neodymium magnet, preferably an N52 grade, as many gemstones exhibit only weak paramagnetism.
- Prepare the Stone: Ensure the stone is clean and dry. For smaller stones, placing them on a flat, non-magnetic surface (like a piece of paper or plastic tray) can help observe subtle movements.
- Perform a "Pick-Up" Test: Gently bring the magnet close to the stone. If the stone lifts off the surface, it indicates a strong magnetic response.
- Perform a "Drag" Test: If the stone doesn't lift, try placing the stone on a non-magnetic surface (e.g., a cork or a thin piece of plastic floating on water) to reduce friction. Slowly bring the magnet near one side of the stone. A "drag" response is observed if the stone moves or rotates towards the magnet.
- Observe the Response: Note whether the stone strongly sticks, weakly sticks, shows a clear drag, or has no discernible reaction.
Table of Magnetic Semi-Precious Stones
| Semi-Precious Stone | Primary Magnetic Mineral / Cause | Magnetic Response | Common Colors |
|---|---|---|---|
| Lodestone | Magnetite (Fe₃O₄) | Strongest (ferromagnetic), naturally magnetized, picks up objects. | Metallic black to dark gray |
| Hematite | Hematite (Fe₂O₃) | Strong paramagnetism; sticks firmly to strong magnets, "pick-up" and "drag." | Metallic gray to black |
| Jasper | Iron oxides (e.g., hematite) | Moderate to strong paramagnetism, especially red varieties. | Red, brown, green, yellow, multi-colored |
| Serpentine | Iron, chromium minerals | Moderate paramagnetism; some cabs stick. | Green, yellow-green, brownish-green |
| Nephrite Jade | Iron minerals | Mild to moderate paramagnetism, especially black varieties. | Green, white, gray, black |
| Rhyolite | Inclusions of magnetite/hematite | Mild to moderate paramagnetism, varies by inclusion content. | Pink, gray, red, often mottled |
| Almandine Garnet | Iron (Fe) | Strong paramagnetism; shows clear "pick-up" and "drag." | Deep red to reddish-brown |
| Pyrope Garnet | Iron (Fe), Magnesium (Mg) | Strong paramagnetism; shows clear "pick-up" and "drag." | Dark red |
| Tiger's Eye | Crocidolite (iron-rich amphibole) | Mild paramagnetism. | Golden-brown with chatoyancy |
Understanding the magnetic properties of semi-precious stones can be a fascinating aspect of gemology, offering clues about their internal composition and unique characteristics.
