In this area it is quite common for people to take things abandoned at the curbside: furniture, kids' old, outgrown yard toys and such. No one considers it theft as it was plainly thrown away. If one is in doubt one can ring the doorbell and ask (for example, a child's bicycle-- the child who lives there may have left it there without thinking.) But generally speaking if it is curbside with other trash, it is being thrown out and can be taken by someone who has a use for it. It's just another way to recycle.
Dumpster diving is prohibited by most businesses because (I would think) of its being hazardous to the "diver" (broken glass etc.) and the potential for liability to the property owner.
The OP didn't say where the trash was where they found the pearls, but my feeling is that the OP was entitled to keep what he found in the trash-- since it was in a bag with with junk watches, it seems the owner disposed of it for reasons of their own. We had a number of items we threw away after cleaning out my father-in-law's home after his death; broken bits of costume jewelry and such. It is possible this was true of the person who threw these things away. Maybe they did not like the pearls and did not want to be bothered with them. It is not as though the pearls were in a sack with a quantity of obviously valuable items-- if so, then I might wonder if a burglar had dumped the sack to escape being caught with the items on his person. But a little bag containing mostly junk? Trash.
On the other hand, I would see valuable jewelry found on the street as something to try to restore to the owner if at all possible, whether by advertising in the Lost and Found ads or by taking it to the police department. The police however would not make any effort to find the owner of (say) a gold chain -- they would just wait to see if someone reported losing it.
