What is "rare" about rare-earth magnets?

I can't open a woodworking magazine, catalog, or website without seeing rare-earth magnets used in everything from tool organizers to shop jigs to jewelry boxes. They seem to be everywhere, and not too expensive, so where does the term "rare" come from?
—Andrew Rhinehart, Dearborn, Mich.

The magnets you refer to, Andrew, get that name because they contain neodymium or samarium, two of 17 rare-earth elements. Despite that designation, some of those elements can actually be as abundant in the earth's crust as lead or tin. The word "rare" applies to them because they don't appear in concentrations or seams, as do elements such as copper or silver. Instead, rare-earth elements occur widely dispersed in ores; low concentrations often make mining them economically unviable.

The rare-earth magnets you find widely available are an alloy of neodymium, iron, and boron. Neodymium magnets were invented in the 1980s, but it wasn't until their prices dropped significantly in the 1990s that they made a splash in the woodworking world. Today, you'll find them in many applications that benefit from strong magnetism in a compact form, including cordless-tool motors, hard-disk drives, magnetic hold-downs, jewelry clasps, cabinet latches, tool holders, speakers, MRI machines, and wind turbines.

Compared with other types of permanent magnets, such as ferrite (also called "ceramic" and composed mostly of iron with a few other elements) or alnico (iron with aluminum, nickel, cobalt, and other elements), rare-earth magnets can be 10 times stronger.

All rare-earth magnets receive an "N" rating (N stands for neodymium), designating their strength. The strongest—N52 magnets—are noticeably stronger than N35 magnets. But that doesn't mean N52 magnets are best for all applications. An N42 magnet that's slightly larger than an N52 magnet may have the same pull but be less brittle and cost less.

Rest assured that your rare-earth magnets will perform long into the future; neodymium magnets lose less than 1% of their magnetism every 10 years (provided you don't expose them to temperatures exceeding 175ºF).

With all that power comes possible health hazards, however. Rare-earth magnets can interfere with pacemakers or other implantable devices. Swallowing them can lead to serious health problems or death if they attach to each other through intestinal walls. When using neodymium magnets in your projects, always secure them within a sealed cavity or attach them solidly with screws or epoxy. Rare-earth magnets will corrode readily, so don't subject them to applications that wear down their factory-applied plating or coating. They're also brittle, so avoid having them forcefully collide with each other or ferrous surfaces.

Today, the majority of rare-earth elements are mined in China, partly due to that nation's low cost of production. (It was China's entry into the market that brought down prices in the 90s.) Because these elements occur in low concentrations, extracting them from ore can be especially damaging to the environment. For strategic and ecological reasons, research is underway to find substitute materials.