earth layer significative element

Hello Everyone,

s I'm studying for my final in geochimie I find hard to find which elements are significant and relevant when it comes to separate layers of earth. ( aside from the obvious things like oxygen in atmosphere... ). In other words, if you could get a random rock from one of those layers, what are the elements that will surely indicates from which one in comes? 


Any Idea , Suggestions would be appreciated,


I didn't find the right solution from the internet.


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