Minor correction: “steelies” were plated with zinc rather than nickel, which was also a strategic metal.

The zinc plating proved to be unsatisfactory because it quickly wore off in spots, allowing the underlying steel to rust; the rest of it reacted with moisture, skin oils, etc. and turned an ugly gray. The gray color also led many people to erroneously believe the coins were struck in lead, a misunderstanding that persists even today.

erv is right. These are a very famous error, though as the link says the regular (steel) 1943s are of very low value. Incidentally, the mint went back to copper in 1944, but a few steel cents were made in error. While rare, they don’t have the cachet of the ’43 coppers. Anyway, interesting material though the chance of finding any in circulation is zero.

It is rare as the 1943 penny was struck in steel due to the war resulting in a shortage of copper. A few were struck in bronze. http://www.pcgs.com/coinfacts/category/half-cents/lincoln-cent-wheat-reverse/type-2-steel-1943/1613

1943 pennies were never supposed to be made with copper planchets. They were all supposed to be made of nickel plated steel, to conserve copper for the war effort. A very small number of copper pennies were minted by mistake and that is why they are so special and rare. The steel pennies are not worth the metal with which they were minted, but the copper ones are worth a small fortune.