1929,1930 & 1968/D aluminum colored cents.

Hello again everyone. Im just too curious to seek info. Why these 3 cents seems to have a different color like of an aluminum.  I have many lincoln cents and as old as them but they dont look like them.

the wheat ears cents weights ave.3 grams each while the memorial cent weights 3.5 grams.

all 3 doesnt sticks you a magnet too. Could this be just a result of aging process? Chemical exposure? 
i simply hope someone  can help me identify them.

colors sometime brought additional features to a cents.

thanking you all again in advance.

Looks like they have been plated with something.

Perhaps but they dont look shiny? I have encountered local silver plated coins and they are terribly shiny. They are used on wedding ceremonies here in the philippines. We call them “ aras “. For good luck!

 Dated 1929 1930 1968 - so between then and now some of the shine has vanished. 

Such as if plated in 1969 that is 56 years ago, for example. 

Well, i can consider that analysis. Maybe even gone thru a lot too. finally, nothing seems to be unusual.

everythings clear now. 
i dearly appreciate your time & thanking you all again.