Metal Detector

Informational accessories work from your inventory or your Void Bag. The only reason to give it a real slot is if you want a modifier on it, which almost nobody does. In multiplayer, teammates standing near you get the readout as well, so one person carrying it covers a small group.

You get one line at a time, naming whichever detected tile is worth the most. That means a Life Crystal or Life Fruit sitting in range will hide every ore around it. Keep collecting those even after you have capped your health, otherwise they permanently mask the readings you actually care about.

The detection area is a rectangle centered on you, which makes it possible to bisect. Walk left until the message stops and mark the spot, then walk right until it stops and mark that. Your target sits somewhere on the line between the two marks. Repeat vertically and you have a point. A Compass or a Depth Meter makes those marks exact instead of eyeballed. A Spelunker Potion or Magic Lantern finishes the job and catches gems and lower tier ores that the detector will not mention at all.

Hellstone is not detected, presumably because the Underworld is built out of it and the message would never stop. It does report plenty of things the tooltip never promises, including pots, fossils and Strange Plants.