"Collect them all and..."
Silver Coins ( 銀貨 ; Ginka ) are collectible coins with a great value that are scattered all over Istrakan, in Boktai: The Sun is in Your Hand. There are 30 coins in all, and collecting all of them unlock the sound test in the main menu.
All of the coins are inside in green chests, some of the chests are invisible them that requires a See-all Nut or a Scan Grenade to see, and other chests appear when an puzzle is solved.
In Boktai 3: Sabata's Counterattack, when talking to kid several times he will ask where are the silver coins, in that, django answers that he left them in istrakan.