There’s more than seven. I count 11 (or 10, if you don’t count Silver K, since it was a hoax), not including the (as far as I’m concerned) non-existant "Purple Kryptonite
(I’m quoting my own post here, 'cause I don’t feel like retyping it from the “ask the Comic Guy” thread)
The five major varieties
#1: GREEN KRYPTONITE
Origin: When Krypton blew up, (almost) anything not vaporized became Green Kryptonite
Effect: Kills superpowered Kryptonians (over the course of hours/days depending on amount, proximity and strength). The first stage of Green-K poisoning is the immediate reduction of powers (the loss of powers gets worse over time) accompanied by pain, As the Kryptonian gets close to death, they begin to sweat, possibly halluncinate and in the very final stage, turn green.
It does not effect Kryptonians without superpowers, despite a few stories to the contrary. It also has no effect on humans. Note: quantity matters (more is more dangerous) and different pieces of Green-K have different levels of power.
Also note that despite what you may know of normal radioactive elements, nothing but lead and distance can stop K-radation. All types of K-radiation can be stopped by lead, but gold, which is better at stopping normal “earth” radiation doesn’t protect against the effects of K-radiation.
A) There is some debate on whether or not Green-K (as well as Red-K, see below) is invulnerable in an Earth-like environment. I fall into the “no, it’s real tough, but it’s not invulnerable” camp, but there is quite a bit of evidence for both sides of the debate.
Note, its been explicitly stated many times that Green-K is immune to friction heat (but not heat vision…note that I think that this is silly, but it was said over and over) the reason being that “Kryptonite can’t combine chemically with Oxygen, which causes combustion!”
B) Yes, there is a lot of it on Earth. Baby Kal’s rocket had an experimental warp drive which ripped open a hole in space and a vast quantity of Green-K came through
C) Unlike normal radioactive substances, the damage caused by Green-K is not cumulative. So each time Superman is rescued from the final stages of Kryptonite poisoning, he recovers at once with no permanant or lasting effects. It may defy common sense, but that was the unequivocal rule for 20+ years. Note: If an exposure to Green-K is long enough to turn the victim green, the victim will die. (Turning green is the last stage of K-poisoning.)
D) There’re also contradictory accounts as to whether the power-loss and weakness that Superman experiences include him losing his invulnerablity (to the point where a normal gun would be a danger to him). I tend to think that his invulnerablity remains till (or even after) death.
#2) RED KRYPTONITE
Origin: A (large!) swarm of Green Kryptonite passed through a space cloud and changed into Red-K.
Effects: Red Kryptonite causes strange, unpredictable and (strangely) exclusively non-fatal mutations on superpowered Kryptonians (it doesn’t change a Kryptonian into a rapidly dispersing cloud of vaporous kryptonite particles) . These mutations are usually physical in nature (Superman becomes a giant gorilla, gets an ant head, hair and nails grow wildly). Occasionally the effects are power related (causes heat vision to turn on permanently, makes effectee immune to Green-K ). Rarely the effects are psychological (Superman hallucinates that he’s back on Krypton, thinks that he’s Superbaby, etc), Regardless of the effect, Red-K 's effects wear off in a 24-48 hour period of time. Note that despite one story to the contrary, Red-K cannot affect humans.
A) Each piece of Red-K has a particular effect. IE: If a piece of Red-K makes Superman swell up like a balloon, it will have the same effect on Krypto, Jax-Ur, Supergirl etc.
B) Each piece of Red-K has a unique effect. (There aren’t two pieces of Red-K that’ll make a Kryptonian swell up like a balloon). Caveat: In later stories, the writers exploited a rule loophole: No two pieces of Red-K have exactly the same, identical effect, so while there’s only one piece of Red-K that’ll make Superman swell to 3 times his normal size, there might be another piece that makes him swell to 3.0001 times his normal size. I’m not thrilled with this, but they published it.
C) Despite a few stories to the contrary, Red-K 's effects do not last exactly 24 hours (based on the vast majority of stories). You cannot set your watch by a Red-K exposure.
D) Once a Kryptonian has been exposed to a particular piece of Red-K , he or she can never be affected by that particular piece again (but that same piece affects other Kryptonians)
E) If a piece of Red-K is broken up, it “counts” as the same piece (each fragment has the same qualities as the original and a Kryptonian immune to the “parent” piece is immune to the fragments.
#3) WHITE KRYPTONITE
Origin: It was created when another swarm of Green-K went through a different space-cloud.
Effects: Kills plants. All plants. Kryptonian or earth, superpowered or not. One of the only three Kryptonite variants that will affect non-super types
A) White-K will kill any and all plant life, Kryptonian, Earth-based, sentient (Swamp Thing) etc…except intestinal flora (we regularly saw people handling the stuff without dying)
#4) GOLD KRYPTONITE
Origin: Just a variant of Green-K . Created in Krypton’s explosion (someone suggested that it’s the remnants of Krypton’s Gold Volcano. I LIKE this idea!)
Effects: Permanently removes the superpowers (or the ability to gain superpowers in the case of non-powered Kryptonians) from any Kryptonian exposed. Gold-K radiation has a very short range.
A) Gold-K is rare. It’s the second rarest of the five major types of Kryptonite. We’ve only seen about 4 Kryptonians (in non-Imaginary Stories) affected by Gold K: Quex-Ul, Jay-Ree and his wife Joenne, and a Kryptonian wrestler guy in Superman #204 (plus a clone of Superman in Action #500)
B) There’s a debate about whether the loss of superpowers is inheritable (bias alert: I think it is). Those who think it is say that in Superman #179, Jay-Ree and Joenne explicitly state that their kids won’t have superpowers either and rather than spread this genetic damage throughout Kandor, they choose to be exiled on Earth (and live in a doll-house in Jimmy Olsen’s apartment).
Those who think it isn’t inheritable say that acquired characteristics can’t be passed on, and besides, Jay-Ree and Joenne weren’t geneticists, so they could have been wrong. The debate continues.
C) There’s another debate regarding the origin of Gold-K. The story in Superman #179 has that piece of Gold-K being created when a talking piece of Red-K (don’t ask) is zapped by a nuclear ray from a passing satellite. From this story a number of sources claim that Gold-K is created by irradiating Red or Green-K. I don’t buy this. The other pieces we’ve seen weren’t irradiated in that way, and that particular piece of Kryptonite (the sentient one) was unique in that it had already been Green and Red-K (and I’m pretty sure that it later became White-K). Besides, if all it took was a nuclear zap, Luthor would have been manufacturing the stuff by the truckload.
D) The stories are unclear as to how close a Kryptonian must be for Gold-K to affect him/her. Either they must be extremely close (a matter of inches) or it must touch them. Gold-K’s effects are, however, an all-or-nothing deal. Either the Kryptonian is affected and loses all his powers, or he isn’t and loses nothing.
#5) JEWEL KRYPTONITE
Origin: The final one of five major Kryptonite types. By far the rarest of the five major types, only one piece exists. Jax-Ur went back in time and carved a chunk of the Jewel mountains into a shape that would survive Krypton’s explosion. That one piece eventually reached Earth.
Effects: Jewel-K has no effect on anyone. What it does is allow the telepathic inhabitants of the Phantom Zone to focus their telepathy and blow things up.
A) Jewel-K may be a telepathic lens for any telepath, not just those in the Phantom Zone
B) The one piece of Jewel-K has been broken into two pieces.
Minor varieties
#6) ANTI-KRYPTONITE
Origin: Debateable: It may just be a normal radioactive Kryptonian element.
Effects: Kills non superpowered Kryptonians the same way that Plutonium kills people from Earth. Effects are cumulative and rapidly fatal.
A) Argo City’s foundation turned into this stuff after Krypton exploded.
B) This stuff may also affect humans. Since we’ve never seen a human exposed to it, we don’t know, but it’s a persistant fan theory that I like.
#7) KRYPTONITE X
Origin: Supergirl accidentally created this variant (As far as I know, there’s only one piece running around) while searching for a Kryptonite cure.
Effects: Gives cats (remember Streaky?) and humans superpowers equal to Kryptonians. The effect is temporary and the person (or cat) affected may or may not be vulnerable to other types of Kryptonite.(I seem to recall that Streaky was affected by Green-K once.) In addition, Kryptonite X affects super-powered Kryptonians just like Green K does. (it makes them weak and sick and will eventually kill them.)
#8) SLOW KRYPTONITE
Origin: Only appeared once (in Brave and Bold(!) #179) Accidentally created in by an Earth scientist in an unrepeatable experiment.
Effects: May affect Kryptonians like Green-K does, but definitely affects humans as Green-K does Kryptonians. (makes humans weak, they sicken and eventually die). The effect is non-culmulative on humans.
#9) BLUE KRYPTONITE
Origin: Superman put Green-K in front of a Bizarro Machine and got Blue-K, an imperfect duplicate.
Effects: Blue-K affects Bizarro Kryptonians as Green-K affects normal Kryptonians. (The only Bizarro Supergirl ever created died from Blue-K poisoning) but Bizarro Humans (Bizarro-Lois Lane or Bizarro Perry White, for example) are immune.
#10) SILVER KRYPTONITE
Origin: Despite the famous cover, there’s no such thing as Silver-K. It was a hoax created by Superman’s friends to keep him busy while they set up a surprise party for him.
#11) YELLOW KRYPTONITE
Origin: Luthor created a machine to generate synthetic Kryptonite. Amongst the normal types it created, it also generated Yellow Kryptonite. Superman was busy so a Superman robot showed up quickly. Superman robots were programmed to simulate being affected by Kryptonite so as to maintain the illusion that they were Superman. The robot kind of freaked out when it saw the Yellow-K since it didn’t know how to act in it’s presence.
Effects: If Yellow-K has any effects other than confusing Superman Robots, we’ve never seen it.
#12) PURPLE KRYPTONITE
Origin: Net legend. Doesn’t exist. At least, no-one’s ever shown me a reference. Allegedly it does to Bizarros what Red K. does to Kryptonians. I’ve tried to find a reference in a comic-book to this stuff for years, but never have. I have read every issue of Superman, Superboy, Action, Lois Lane and Adventure Comics (featuring Tales of the Bizarro World) from at least 1960 forward, and in many cases as far back as 1954 forward. I’ve read most of the Jimmy Olsens and World’s Finests from that period too and I’ve yet to actually see Purple K (granted, my memory could be faulty or I could have missed it)
I’ve come to the conclusion that maybe it was mentioned in a letter column somewhere, but it never appeared in a story. If anyone can reference a title/issue number, I’ll change my stance.
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Ace
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Metamorpho says “NO!” 
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Alfred Bester, who also wrote the GL oath. In addition, though, I’m pretty sure that Otto Binder wrote a few as well.
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Because Alfred was blown up and turned into the super-villian “The Outsider”. Or, because to some moron on the TV show’s staff, it seemed less “Gay” to have a woman around the house too.
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Slam Bradley? Zatara?
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I have no idea!
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Bill Finger
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By being murdered by a League of Assassins candidate who was on a try-out. Th’ guy had a hook.
Great set of questions, **Chuck![/]b and one right back atcha!
What was the name of the strongman in the circus where Boston worked?