I'm out trying to figure out what type of rocks give tones on the metal detector but crush up into silver powder.

The metal detector makes sound like gold but once the rock is crushed up no metal is found. There is pyrite in the rock also and nothing magnetic.  I was hoping the Falcon MD20 would do a better job at discriminating between rocks and metal.

Does anyone else have a hard time telling the difference between a hot rock and gold in the rock?

See my video; https://www.youtube.com/watch?v=XM1hELXCmMg&feature=em-share_video_user

Kevin