What kind of rock is detectable?

[twister]

I was out this morning on a small mountain and wasnt really getting any signals then I got a faint signal so I dug it and eventually I noticed it was a small rock that also made my pinpointer sing. So I checked more and after everything it was a area about 3 feet long and 12" wide that had a bunch of these rocks. Any idea what it possibly be or where I could find out possibly what they are?
Thanks

 

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[Diggit]

LOTS of rocks are like that, usually because of iron, but a lot of other metals. Unfortunately without a description or a picture, nobody will be able to help you much

 

[boris]

I have been told by jewelers that obsidian does this{black like silver}

 

[Charlie P. (NY)]

About any mineral with iron, magnesium, nickel, aluminum . . .

In fact, any metallic mineral or ore of that mineral. Say 10% of anythying solid on earth at room temperature.