What's Kryptonite?

What's This? Poison rocks.

The facts: Whatever caused Krypton's destruction turned the planet's minerals into radioactive material then expelled into the galaxy, much of it following the same path as baby Kal-El's rocket to Earth. As Superman's powers grew, giving him a special weakness that would only hurt him (and all Kryptonians after him, of course) made narrative sense, but the idea first appeared not in the comics, but on The Adventures of Superman radio show in June of 1943. A piece of Green K wouldn't appear in a comic until Superman #61, six years later (cover-dated November 1949). To be fair, Jerry Siegel had written about Kryptonite (or, at least, a life-draining mineral from Krypton) in an unpublished story back in 1940. But once it was in comics, it was everywhere, and starting coming in various colors, each with a different effect (see above). The main varieties are Green (power drain and fatal), Red (never the same weird effect twice), and Blue (healing). There was so much Kryptonite around in the Silver Age, that an important storyline in the early Bronze Age destroyed much of it. The Crisis would reset the varieties back to Green anyway (Red would be a Mxyzptlk invention), but the number has grown once again.
How you could have heard of it: It's since passed into the vernacular. It's become the equivalent of "Achilles' heel" in modern parlance. Kryptonite has been an element of every Superman project in other media, and recent appearances in the comics include Superman: The Kryptonite Spectrum, a non-canonical maxi-series that explored the many colors (by the team that brought you Ice Cream Man); and Dan Slott's Superman Unlimited run, in which Kryptonite becomes so common, a whole city is built out of it, leading to shenanigans.
Example story: Superman #173 (November 1964) "Tales of Green Kryptonite No.1" by Otto Binder and Al Plastino

Very weirdly, the Silver Age gave us a chunk of SENTIENT Kryptonite that could tell its own stories (I think there were four "Tales of Kryptonite" in all) and become the Forrest Gump of Kryptonite. Like ALL Kryptonite, it started life as a piece of Krypton, specifically, a statue made of rare illium (do only CERTAIN minerals become Kryptonite, or else it would be too common?). That's where it first "met" Jor-El, his family, and his pets/guinea pigs for space flight.

When Krypton exploded, the head of that bust (presumably, of a famous Kryptonian scientist) broke off and was propelled into space, transformed into Green K. (We're reminded by an editor's note that some of this Kryptonite goes through space clouds that changes its color and nature.) In any case, our friendly narrator finds itself on Earth, in Africa, where a native puts it in the eye socket of his religious idol. After a treasure hunter steals the OTHER eye (a 500-karat diamond), a certain Super-Monkey imitates him and pries loose the Green K eye.

It makes Beppo feel all funny and he almost gets eaten by a big cat, but Green K rolls away and Beppo is saved. It's eventually found by a geologist who ships it back to America, but the plane crashes near a farmhouse, and you can see it's already THINKING. No explanation as to when and how it became sentient (and, I suppose, retroactively remembered everything prior - was it sentient when it was sculpted into a bust? Is it a property of illium?). It's also quite good at understanding the alien world around it. That winter, it's found by the poor farmer's widow, who soon gets visited by a kindly neighbor bearing a basket of food for the woman and her children. That neighbor? Martha Kent, with toddler Clark in tow.

He's hurt by the lady's new Christmas ornament. And it seems Green K recognizes the boy. After Christmas, it's thrown out into the front yard again, and relieved to see the Kents moving to Smallville proper, not so near its deadly rays. But Professor Lang is doing some archaeology real close to home, finds the rock, and decides to put it in his classroom collection. Green K realizes that he might give Superboy's identity away, but nothing happens. A new rocky hero enters the story...

Lead! A Kryptonian's best friend! Later, Lang got rid of his geological specimens and gave them to his daughter Lana. One day, Clark goes to her house to see if she wants to go see a movie, but there are burglars in the house and they knock Clark out and throw him in the pool (grisly!). He plays along for the sake of his secret identity, but then realizes he's really drowning! Yeah, Greek K is in the rock garden nearby! So Superboy detaches his cape so it floats up and acts as a signal for Krypto. Obviously, he, too, is affected, but dogs are great at digging, so...

Superboy uses massive lead tongs to snatch the Green K, and he brings it to Lex Luthor, who is not yet his mortal enemy. Lex doesn't manage to create an anti-Kryptonite serum out of it, but when his lab catches fire (it's not the day he loses all his hair, however, he's away) and Superboy must risk the Green K radiation to put it out. It's the Silver Age, so irony incoming.

That's right, some random experiment rendered the Green K inert, but Lex never finds out, and Superboy throws the rock to the Arctic before it has any chance of harming him. And so the solution to Green K radiation poisoning is lost... forever. 

A part of me DID want to cover a story for each Kryptonite type, but I've already covered most Red K stories under the Reign of the Supermen feature, and I didn't really want to be stuck in Silver Age Superman stories for weeks on end. So we're moving on...

Who's Next? The Spectre's archenemy.