Answer:
I'm assuming you're talking about something like a regular housekey, in which case there are numerous things you can describe!
Its mass: how much it weighs, times the gravity of the system.
Its color: is it silver-colored, or gold, etc.?
Its volume: you could probably calculate this, though it would be hard because it's not a traditional shape, I assume.
Its density: this is mass/volume. It means how many of the atoms are in a given space; how tightly packed the atoms are.
Its tendency to rust / its specific combination of metals or alloys: is it made of copper, tin, steel.. and these are all things that can rust to varying degrees.