I thought that Kachink was the metal cube that Supreme Kai had materialized. I believe the z-sword was broken on the Kachink. I don't recall it being said that the z-sword was made of Kachink.
I think you may have misinterpreted the article (not trying to put you down, as it could also be a misinterpretation on my part, and not yours). You are correct though, it is the cube, not the Z-Sword that is made of Kachin, and unless I am the one that is mistaken, the current page does in fact state that the cube is composed entirely out of Kachin, and makes no mention of what material the Z-sword is made out of.
I'll go into a bit more detail, that hopefully will help make it more clear: The only time we see Kachin in the series is when Supreme Kai summons a large cube of it, hands it off (using telekinesis) to Goku. Then, Kai goes on to briefly explain that the cube he summoned is made of Kachin, and that it "is said to be the hardest metal in the universe." After briefly testing the metal with a knuckle, Goku then tosses it at Gohan, Gohan swings the sword like a baseball bat, hitting the cube with the Z-Sword which breaks the sword roughly in half. As for the sword itself I don't believe it was ever stated what material it was composed of, I also don't believe Supreme Kai knew what the sword was made out of either, he had just simply heard the prophesy that it would bestow great power to whoever could remove it from the rock. Leading him (and the audience) to interpret that the sword itself is a mighty weapon, instead of its actual purpose, trapping Old Kai.
The episode in which all of my above description takes place in is (unedited) Episode 250: "Out From The Broken Sword." I'm not sure if any of this info is contradicted/different in the English dub, so for the record I got all of my information via the Japanese dub, with English subtitles.
-Trunksu 07:08, February 23, 2010 (UTC)
Density[]
Theoretically, if it was that dense, Goku wouldn't be able to lift it. A teaspoon of a white dwarf equals 15 tons and a teaspoon of a neutron star is equivalent to the mass of 900 Great Pyramids of Giza. Goku couldn't even move with 40 tons on each hand, and thus a cube made of the true densest material in the universe wouldn't be liftable. ~Hyper Zergling 21:19, October 29, 2010 (UTC)
Except the 40 tons thing is a gross in consitency with what Goku has done both before and after Hadomaru 19:27, September 3, 2011 (UTC)
That's all irrelevant anyway, because it said hardest, not densest. They don't mean the same thing.