A lodestone, or loadstone, is a naturally magnetized piece of magnetite. They are naturally-occurring magnets, which can attract iron. Ancient people first discovered the property of magnetism in lodestone. Pieces of lodestone, suspended so they could turn, were the first magnetic compasses, and their importance to early navigation is indicated by the name lodestone, which in Middle English means 'course stone' or 'leading stone'. Lodestone is one of only two minerals that is found naturally magnetized; the other, pyrrhotite, is only weakly magnetic. Magnetite is black or brownish-black with a metallic luster, has a Mohs hardness of 5.5-6.5 and a black streak.
The process by which lodestone is created has long been an open question in geology. Only a small amount of the magnetite on Earth is found magnetized as lodestone. Ordinary magnetite is attracted to a magnetic field like iron and steel is, but does not tend to become magnetized itself. Recent research has found that only a variety of magnetite with a particular crystalline structure, a mixture of magnetite and maghemite, has sufficient coercivity to remain magnetized and thus be a permanent magnet. One theory suggests that lodestones are magnetized by the strong magnetic fields surrounding lightning bolts. This is supported by the observation that they are mostly found at the surface of the Earth; not buried at great depth.
Among the first references to lodestone's magnetic properties is by 6th century BCE Greek philosopher Thales of Miletus, whom the ancient Greeks credited with discovering lodestone's attraction to iron and other lodestones. The name "magnet" may come from lodestones found in Magnesia.
The earliest Chinese literary reference to magnetism is in a 4th-century BC book called Book of the Devil Valley Master (鬼谷子): "The lodestone makes iron come or it attracts it." The earliest mention of a needle's attraction appears in a work composed between 20 and 100 AD (論衡): "A lodestone attracts a needle." Medieval Chinese navigators were by the 12th century using lodestone compasses.
The American astronomer John Carlson based on his finding an Olmec hematite artifact in Central America suggests that "the Olmec may have discovered and used the geomagnetic lodestone compass earlier than 1000 BC," thereby predating "the Chinese discovery of the geomagnetic lodestone compass by more than a millennium". Carlson speculates that the Olmecs for astrological or geomantic purposes used similar artifacts as a directional device, or to orientate their temples, the dwellings of the living, or the interments of the dead.
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