Answer:
density
Explanation:
It won't be color. Many metals have the same color although if Greg was handed copper or gold they have pretty distinctive colors.
Not many metals burn. It depends on what you mean by burn. The alkali metals will burn when in contact with water. Not many ignite if you put a match them them.
Rusting is a singular property. I very few common ones rust.
I would pick density. Few metals have the same density.
