Herr-Eisenheim

I am unique, for I am never the same I am but everchanging, for that's part of the game Careful, hold me for long and I change my form It's not because of you- this is the norm Hide me away, and I start to decay Ignore me for long, I'll make sure you pay I am not true, but I was never a lie Can you guess what I am? Give it a try! You share me with others, you keep me alone Sometimes you'll notice that I might have grown Do you understand this? Or is the answer no? Or does a clue, perhaps, clearly show?