I think the problem, with detecting publicly owned lands - stems from the fact that 'finding valuable jewellery' is a distinct (if remote) possibility!.
This article probably gives a clue.
Theft by finding? It's in the bag | SBS News
Theft by finding? It's in the bag
What it alludes too - is that by law - its not 'finders keepers' in relation to private property.
Here at least there is a law called "stealing by finding"!
When you FIND valuable property (like personal jewellery items) - your obligated by law to hand them in to local police as lost property. You are given a receipt for the found property you hand in! At the end of a specified period (3 months from memory), you are contacted and eligible to THEN claim the lost property as your own.
All the lost property that's handed in and not claimed after the 3 months period, goes to the police auction.
So when people see detectorists searching parks and beaches etc - SOME folks assume that its being done for "personal gain" (treasure hunting) NOT legally as a recovery of lost property search.
That's why people do stuff like ring Police etc when they see someone detecting a public park. The believe (correctly) that anything on a public park is unlikely to be your own private property your searching for - your searching for something lost by some other citizen and still legally owned by them!.
Property laws USED to be a big thing - you were hung for stealing a horse. Car theft is still serious felony crime!
"Hey look what I found metal detecting the local car park - a '57 Chevy!" - is hardly going to wash as an excuse, when the local Policeman stops you driving a stolen car!
What difference if it's someones prized wedding or engagement ring?
Finding someone else lost property does not make it your personal property. This is why Pawn shops ask "how did you come bye it?" when buying items - they have an obligation to determine it if was legally owned first to be able to buy it under the protections of the pawn brokers act. (The Item goes in the buy room for 3 months and the local police have the right to search the buy room looking for items reported stolen). If the Pawn broker buys something that is reported stolen, and found in the "buy room" they lose it and the money they paid out to buy it!.
I know everyone here likes to post up picks of their latest treasure find... but private property like jewellery - isn't classified as "treasure" for the taking.
Real treasure (shipwreck stiff for e.g.) is legally defined as belonging to the State & its a BIG offense (with biog fine and incarceration) to take or keep it.
I think this is where detecting on public land for someones lost property differs from say finding a gold nugget (mining) a totally different legal construct.
I also think this might be WHY Police will sometimes take the trouble to talk to people, detecting public parks and beaches etc, - coz they know that by finding valaubles and keeping them you are breaking property laws - something they take an oath to upkeep.
They probably don't, fine you - they prefer to take the education approach.
Lets say you find a raw gem stone, and you also find gold nuggets.
You take them to the jeweler and have him fashion a ring out of it by getting the gem faceted & ring cast & then mount the cut polished gem - you did FIND the components of a ring, and by having one made - it becomes your legal property, for which you have a receipt (for the manufacture).
That's different to finding someone else's lost ring and keeping it!
As I understand it the ONLY way you can legally keep such a ring is to hand it in to Police and claim it back after the specified waiting period - at which point you get a receipt for it from the local police and it LEGALLY becomes yours!.
I find myself often pondering this very question, when I see finds of the day posted - where valuable jewellery etc is found and pics posted....
Indeed should the forum even allow such posts unless there's a receipt of ownership from police lost property section?
All good questions.