UPSC CDS 1 2021 GK Paper

Iodine is a non-metal, but it is lustrous. It's because iodine going down the size of the atom is increase and shielding effect of the atom and outermost electron decrease. So that iodine becomes excited by absorbing heat energy, and they emit light. So iodine appears to have a lustrous surface but this element is not a metal.
Note-Carbon is also showing lustrous but only in certain forms of diamond, and graphite.