need help with Rock ID

[fundad]

ok well first of this was my first dig. just purchased a garret 250 ace. I like it. anyhow I was working a rocky shore line.. very hard as rocks beep all the time with this thing. but this beep was different.. and kept on beeping as I hovered.

this rock is really light and im assuming porous.

here are pics. any info will help! thanks all

Mark

 

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[tamrock]

I'm thinking its a piece of slag.

 

[fundad]

any way to find out age?