John and BCD are right. It could be a meteorite, or something of lesser interest.
I have found all kinds of goofy rocks over the years, "hot" and otherwise. Mineral anomalies of all kinds exist and they don't have to be positively conductive to cause a detector to react. I recall one area that was strewn with rocks, making my detector signal, fade and whine like a whipped puppy. As far as I could tell the rocks were actually negative, but my detector reacted to them in all sorts of audible ways.
Garretts are, in fact, somewhat notorious in my experience for responding over non-standard things.
It was one of the first things that I noticed - and had problems with - when using Garrett instruments. Coming from other, more analog detectors, I was immediately struck by their binary, or "digital" nature. Flatly put, all the processing and computin' going on in them made it difficult to interpret what I was hearing.
You still get a lot of that from Garrett newcomers; comments about falsing and digging too much trash and so on are common. Until they learn to work the DISC properly and keep the SENS under control, there are problems sorting out the responses.
As for your lemon hot rock, well... it could be anything. As John says, have it checked out. It may be something valuable.
But I go by a rule of thumb: "Most truly valuable things are, indeed, rare or hard to come by. The odds are not in your favor."
So it is most likely as BCD suggests - just some whacky lump that your detector likes.