Hot rock even with discriminate on?

[eureka77]

I have an teknetics alpha 2000. I'm always tired of digging nails and trash so I turned the dicriminate on to rule out iron, zinc, and foil. Anyway while going along my detector goes off with the highest pitch for my detector, saying 50 cent. Usually it's a soda can or something big, but always trash. I go ahead and dig and it turns out to be this heavy rock.
Why would this set off my coin detector? Thinking something's good inside I broke it perfectly in half. Now both sides go off with the same high pitch. I thought only iron rocks were hot rocks?
Any ideas what this is?
Also how can I break it up further with out losing a bunch of pieces if it's worth breeaking up further?
Thanks for any input.

 

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[eureka77]

Forgot to say also that the sensitivity is turned all the way down to its lowest setting of 1 bar, and a magnet will not stick to it.