Cast iron is much heavier than cast aluminum. A magnet or compass needle will be attracted to cast iron, but will not be attracted to cast aluminum. Cast iron quickly develops a dark coating of iron oxide (rust) after being exposed to humid (damp) air and will continue to rust if left exposed, while cast aluminum becomes coated with an amost invisible whitish layer of aluminum oxide that protects it fom further oxidization. Cast iron is somewhat difficult to scratch compared to scratching the much softer cast aluminum.
In essence, cast iron has a much higher carbon content (~4%) than what one could define as "iron" either 100% pure, or very low carbon content iron. this makes cast iron much harder than many other types of iron, which makes it less usefull in places where the material will have to take a varying load, but also makes it suitable in instances where hardness is desirable - such as Victorian knives. however, modern alloys have made much cast iron useless as they combine its hardness with a reduction in is brittleness