Could be the otolith in his head, some collect them.
the blue Pearl and her head
collect all the Pokemon in the national pokedex
collect all the pokemon
A boat, a net and a smithbill- the thing that helps them collect pearls
The game is technically over but you can still collect Pokemon and go to more places.
In diamond? Pearl? Platinum? Sorry, I don't know about Platinum but I know that for Diamond - and I'm pretty sure Pearl also that you have to buy it after you collect a certain number of gym badges.
50,000 to 1000,000 a year depending on how many pearls you collect a day
go to posidens lair and go till u find the clam. be sure to collect air bubbles
they go 100 feet down into the ocean and collect pearl oysters and mussels and then bring them up onto the shore in wooden barrels and open them up and search for pearls inside them.
Cynthia IS the Sinnoh region champion, so to face her you will need to collect all the badges and defeat the Elite Four.
The fastest way is to collect every pokeon in dimond /pearl or platinum then transport every last one to black and white then collect every black and white native pokemon