What a metal detector can and can't find

In one paragraph

How it works

The coil at the bottom of a detector puts a small electromagnetic field into the ground. Metal inside that field disturbs it, and the machine turns the disturbance into a tone. Different metals disturb it differently, so an experienced searcher can often tell a gold ring from a ring pull before digging. How deep it reaches depends mostly on how big the metal object is: a coin-sized thing at a hand's depth, a phone or a bunch of keys deeper than that. Nothing without metal in it registers at all.

Reads well

Things a detector finds easily

Rings: Gold, silver, platinum, white gold and tungsten all read. A thin band reads smaller than a signet ring, and a good coil still hears it a few centimetres down.
Keys: A bunch is one of the loudest, clearest targets there is. A single car key with a battery and a steel blade reads well on its own.
Phones: A battery, a frame and a board. Big, clear, and no different to the machine whether the phone is dead or alive.
Watches: A steel or gold case on a metal bracelet is a large target. A plastic sports watch reads weaker: a small battery and a steel back, if it has one.
Drones: Motors, frame, battery pack. Reads from further away than most things on this list; the problem is knowing which field, not hearing it.
Metal-framed glasses: Frames and hinges read clearly. See the next list for plastic ones.

A Retrievly search is for something lost recently. If you find something that looks old or could be archaeological, you don't keep it: you report it to the local museum or heritage authority.

Reads weakly

Things that take longer

Plastic-framed glasses: Only two tiny hinge screws are metal. A detector barely notices them; a careful pair of eyes in a small, well-marked area does better.
Very small earrings and studs: Findable, but slow. The smaller the metal, the closer the coil has to pass.
Titanium: Reads, but quieter than gold or silver of the same size. Say if a ring is titanium.
Anything on a rubbish-strewn site: A detector hears every ring pull and bottle cap too. Busy ground takes longer, not because the item is quiet but because everything else is loud.

Doesn't read

Things a detector cannot find

A stone on its own: A diamond or gemstone that has fallen out of its setting has no metal in it. A detector cannot hear it at all.
Plastic, glass, wood, fabric: A plastic wristband, a bag, a wooden thing, a bank card. Nothing for the machine to react to.
Paper and cards: A wallet reads only if it has coins or a metal clasp in it.

One more that surprises people: a magnet. Magnet fishing pulls up steel and iron, so a steel watch case or a bunch of keys, yes. Gold, silver and platinum are not magnetic and a magnet will not pick up a ring. For jewellery, a detector is the tool.

Where it fell

What the ground changes

Grass, parks, fields: The easiest ground. Things settle to the roots and stay there; a detector hears them from standing height.
Dry sand: Easy too. A ring stays near the surface unless people walk over it or dig.
Gravel and chippings: Awkward, though not because of the signal. A ring works its way down between the stones instead of resting on top, and every footstep pushes it further. Driveway and track gravel carries iron fragments too, so the ground is loud. Over a small patch, lifting the gravel and sieving it beats sweeping it.
Wet salt sand and the water line: Harder. Salt water makes the ground itself noisy, so a searcher uses a machine set up for it and works more slowly. The tide also moves things.
Shallow water: Searchable to about waist depth with a waterproof detector, in still water. A current or depth is a different job.
Snow and leaf litter: A detector reads through both. No need to wait for a thaw.
Woods and bramble: Findable through leaf litter; the difficulty is walking the ground, not hearing the target.