My metal detector found a "rock"

[pdsexton]

OK - I'm stumped. I'm hunting in your average Thurston County WA river rock front yard. I got a strong iron'ish signal off this rock, and it drives my Garrett pro-pointer nuts. It's a pretty standard pocket-marked grey rock, more light than heavy. So far I haven't found any others, so it can't be super common. What is it?

 

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[Charlie P. (NY)]

Yep. They are known as "hot rocks". Likely has magnetite in the matrix. Black - as yours appears - usually indicates magnetite. It's a iron oxide (like rust).

Also an indicator that there may be gold in the area where present in large quantity.

 

[gunsil]

Could be coal cinder also. Coal cinders are common around older home sites here, they would throw the cinders on snowy or icy walkways back in the day. I have been driven nuts by them as they are hot rocks too. Cinders are also usually light for their size.