Black pumice with iron?

[UrbanDigger]

I dug a small black stone that appears to be black pumice this past weekend. I was searching in front of a high school near Fort Worth TX. A copper penny and a nail was also in the hole that was only about 2-3 inches deep. I think it's pumice because it's very light for its size and has a porous exterior. The Ace 250 over the stone by itself suggests it has some iron, but the pinpointer response is surprisingly weak. The stone is not noticeably attracted to any magnet.

I've dug 'hot rocks' before, but most of them have been on the heavy side for their size. This black pumice is just not something I typically find when hunting for shallow coins in an urban area. Thinking back on that hunt from a few days ago, I'm now guessing that all of the erratic signals I was getting (Ace jumping around quickly from iron to nickel to near zinc), even on 'good' targets, was caused by rocks like the one I found or other metals in that soil. Turning up the discrimination and adjusting the sensitivity down on the detector was the only way I could reliably get discernible signals from the Ace during this hunt.

I usually blame EMF/EMI for causing erratic signals I get on the Ace, esp when I happen to be detecting close to a cell phone tower or those long-range overhead power transmission lines.

Any comments you might have regarding the stone or the erratic signals in the low-mid range on the Ace would be appreciated.

 

[HuntinDog]

I can't tell how big it is, but could it be a piece of old graphite battery core.

 

[floridabig]

I've had a similar problem before while digging at a cousin's house in Virginia. We both had found with the detector a couple of rocks that went off noticeably but the pinpointer signal was weak. Those stones weren't porous though we ended up cracking them open and the signal just vanished. I don't know a thing about rocks however so I'm just thinking that was some extra terrestrials play with us.

 

[Rock Jock]

Could be a cinder from an old steam locomotive or maybe a low grade coal cinder from a potbelly stove. I've dug railroad cinders as "hot" rocks occasionally and there was no doubt that they were railroad cinders.