Grey sludge that sticks to magnet from lakes?

[DetectorRob]

Have any of you come across this grey sludge that sticks to your magnet from the bottom of a lake?

What is it??

 

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[ecmjamsit]

Magnetite. In Colorado dredgers call it black sand. It is an iron compound.

 

[bowwinkles]

I use liquid car wax on my magnet and this helps to prevent rust plus makes it much easier to clean that type of contaminate.

 

[1637]

would try to rap it with rubber,
brad