The information given is inconsistent, though, and not terribly realistic. Here's what was said:
- The words on the alien's transmission read "Atomic Weight, 08 FE676"
- J'onn says "It appears to be a simple hematite, but iron oxide isn't radioactive."
- The alien replies "Look at the atomic number. 676. It makes no sense."
- Shayera looks at the screen and says "It's tranuranic iron ore. Nth metal, like my mace but unrefined."
First, atomic number and atomic weight (also called atomic mass) are different things. The former is a count of an atom's protons, while the latter counts both its protons and neutrons. It's the atomic number that you see most prominently displayed on periodic charts. An theoretical atomic weight of 676 would equate to an atomic number of about 300 or so.
Either way, that's ridiculously high. Currently, the last element discovered, Ununhexium, has an atomic number of 116, and it has a half-life measured in milliseconds. So the alien is right to say that 676 makes no sense, and it doesn't matter whether he's talking number or weight.
Hematite is the mineral form of iron oxide, so J'onn's statement taken by itself makes sense. And the screen seems to have some bearing on this, since 08 is Oxygen's atomic number and Fe is the abbreviation for Iron. But iron oxide is a compound, not an element, so I don't know how atomic mass could apply.
Scott beat me to the next point. Transuranic refers to elements with more protons than Uranium (#92), which would describe either of the atomic numbers (300 or 676) suggested above. But it doesn't describe iron, which has an atomic number of 26. If an element is transuranic, it can't be iron. Plus, transuranic elements are all radioactive (which contradicts J'onn's assessment) and have only been produced artifically.
Apparently, DC writers in the past have occasionally treated Nth Metal as an alloy rather than an element. Arguably, that's a better approach. And this isn't the first time the DCU has played with real chemistry. John Byrne put Kryptonite at 126 on the Periodic Table. Sure that has its own flaws, but it's a better fit than what they said here about Nth metal.
(And while it's technically a fanfic explanation, CBR's Cei-U! had an intriguing take on Kryptonite. His explanation was that the rock is a compound that contains significant amounts of the element Krypton, and the rock and the planet were named by Earth scientists accordingly. Thus, 'Krypton' is the Earth name for the dead world, and not what Jor-El actually called his home planet.)