Is this a "HOT ROCK"
Can you tell if it looks volcanic or sedimentary in origin? I have seen very dark to black rocks of both types that I would consider 'hot'. On some of the beaches here I have seen rocks of clay origin that holler in all-metal but just sit quiet in descrimination. I suspect this is caused by microscopic magnetite. Volcanic lava and porphries can also do this, but these rarely contain visible free quartz. I have never encountered sulfides (ie pyrite) that make my detector go off, but it isn't a Garrett, so there might be sensitivity differences. Sorry I can't be much more help than this without examining the rock firsthand. You might take it to a college in your area and ask the opinion of someone in their geology dept.