Strange chunk of metal, has NO effect on detector, what is it?

Looks an awful lot like Pyrites to me. Often feels cold to the touch and weighs more than other stones of the same size.
The example from my own collection (above) does not issue a response from metal detectors.

Agreed, it does look like phyrites of some kind.
I find minerals to be a bit like books and people; Not a good idea to judge them by their cover alone., I have some specis that at first glance could be described as opalised wood but are so radioactive at 300,000cpm that you would not want them sitting in yer pocket next to yer nutts for very long.