Actually, SuperRobot, adamantium need not be a new element. It could be an alloy. In fact, I believe that it is supposedly an alloy, created by a metallurgist who was trying to duplicate the metal in Cap's shield (which is even more indestructible than primary adamantium). He is the same guy who created Cap's shield, but the notes he made for the making of it were destroyed if I recall correctly, and he was unable to duplicate it.

Some have suggested that Cap's shield is an alloy of primary adamantium and Wakandan vibranium (accounting for its energy-absorbing and impact-absorbing abilities, which is why it can block a blow from the Hulk [or Superman in a crossover] and not shatter Cap's forearm from the transmitted kinetic force, or how Cap can put it under his feet when jumping from a height that would normally kill him and it would enable him to survive without injury), but the current wisdom is that while it does have aspects of both, it is actually neither.

There is a material that may be possible in the real world that would be almost as durable as adamantium, without being incredibly dense or massive (e. g. neutronium). It would be a phase of ordinary, everyday carbon, just as diamond is. Basically, if you look at the Periodic Table, you'll see that carbon sits in a spot right on the border between heavy metals and non-metals. Even in the forms that we know of, carbon often manifests metal-like properties (graphite is electrically conductive, for instance).

The hypothesis is that, given temperatures and pressures orders of magnitude greater than those needed to turn carbon into diamond, carbon will form an actual metal, perhaps using a buckyball or buckytube-like molecular structure, with resulting very strong molecular bonds. If it can retain that phase once the pressure and temperature are removed, and exist at room temperature, it may indeed be an adamantium-like substance.

Another interesting metal possibility is that hydrogen may be able to form a metal under similar conditions. If it can remain stable at normal conditions, it could not only be an incredibly light but reasonably durable (though not adamantium-like or even steel-like) metal (wonderful for aircraft), but it may even be a superconductor at room temperature!

Kryptonite, on the other hand, would pretty much have to be a new element formed from fusion of heavy nuclei in the explosion of Krypton, with an atomic number probably in the 150 - 200+ range.


TheRockit:

Now I know Spiderman has kicked the Hulk's ass once... he was granted the power cosmic and punched Hulk into orbit... since Hulk doesn't have flight or life support, he basically spun around up there holding his breath until Spidey felt bad about it and rescued him.

It wasn't the Power Cosmic. It was the Enigma Force. Spidey had become the then-current Captain Universe, without knowing it.

To those who don't know: Captain Universe was a very unusual hero in the Marvel Universe. Basically, at all times there is a Captain Universe, but it could be anyone. The Enigma Force (from the Microverse of the Micronauts) endows a person with the Captain Universe power and identity long enough to solve some crisis (personal or otherwise). It multiplies any abilities or talents they may already have by a factor of fifty, plus certain abilities that are consistent among Captain Universes (flight, molecular control, etc.). Since Spidey normally has Class Ten strength (he can press about ten tons), that gave him Class 500 strength. The Hulk is among several beings in the Marvel Universe who average Class 100 strength (Thor, Hercules, Wonder Man, Titania, Sasquatch, etc.). Spidey was thus about five times stronger than the Hulk at that time, though usually the Hulk is ten times stronger than Spidey.

In Spidey's case, the crisis was that a fusion of three of the most powerful Sentinels was formed, the Tri-Sentinel, and only a Captain Universe who was already super being could defeat it. Once he did, the Enigma Force left him and presumably went on to someone else.