Why did this strange grey rock beep with white mineral substance?

[kitt]

Sorry for bad pic my cam just died.
Went to the beach and got about 10 minutes in with my detector and this rock came up under 5cent icon on the ace 250.
I let it soak in some water and rinsed it off and now this is what it looks like. And it still beeps 5 cent under my detector.
I have Identified the mineral as priceite: calcium borate. The rock I still cant figure out why it beeps.

 

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[johnnyblaze]

Maybe iron in it

Blaze

 

[golden sluice]

It might be a hot rock. Or cold stone... check youtube, they talk about these unknown md rocks.

 

[RobRieman]

Melted aluminum?