Consider the difficulty of imparting large circulating eddies on tiny links - the most effective means is with a high freq xmit coil. To get a high Co values the target needs a flat area, like a coin, to drive currents.
Rings can make higher Co values if the band is wider, but if the band is broken the current cannot fully flow and the Co value drops.
Then there's orientation in the ground - a ring laying on its side or a coin on-edge may intersect the coil's more vertical field lines mostly along the edge - where surface area is minimal. But with a bit of an angle in the soil the ring or coin's surface is exposed more and will rate a higher Co value as more eddies can be induced.
If the surface is corroded or pitted, greater resistance decreases currents and a lower Co results. When metals are mixed with gold or silver, the resulting alloys can greatly decrease surface conductivity, making gold rings appear like foil (12Fe, 02Co), even though the original metals were very conductive in their own right.
Thus, it is understandable why most people hunting for jewelry simply must dig everything, unless it clearly is iron. And it is why a higher freq machine excites at depth that poorly oriented, alloyed ring, with the broken band, better by making stronger eddies than a low-freq machines.
