Radiometric dating - creation/evolution question - need all the help i can get!!?

I was having a discussion with a friend about radiometric dating and he asked a question I really didn't have an answer too. "When one picks up an object (say a bone/rock etc.) how does one know which dating technique to use. (Carbon dating is only good for about 50,000 yrs and several other dating... 顯示更多 I was having a discussion with a friend about radiometric dating and he asked a question I really didn't have an answer too.
"When one picks up an object (say a bone/rock etc.) how does one know which dating technique to use. (Carbon dating is only good for about 50,000 yrs and several other dating techniques have varying upper and lower bounds)"
To that I said, "O well we know that the earth is 4.7 billion years old and so we would use (say) x dating method and a dino bone is some million years old so we would use (say) y dating methods."
To that my friend argued that thiss unfair because we are already assuming that the earth is 4.7 billion years old and hence using a dating method that supports that claim is unfair.
So I want to know how do geologists determine what radiometric dating system to use on objects.

Also I would love it if someone could also please explain how these radiometric datig methods are calibrated.
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