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My guess would be that its more apt to scratch seance its touched more often. I would guess the button would be the same material as the rest of the glass.
 
Wirelessly posted (Opera/9.80 (iPhone; Opera Mini/5.0.019802/18.794; U; en) Presto/2.4.15)

Last time I checked glass was hard.... Not soft :p

Definately plastic type material
 
Well each iPhone I have had, there would be scratches on the little button but not the rest of the phone. And now I already have that problem on my iPhone 4.
 
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