The Seven Hidden Treasures, as their name says, are seven secret items have been hidden around Rijon in Pokémon Brown. Nobody knows of their usage yet, but it is mentioned that they will have a special purpose in Pokémon Prism.
The Seven Hidden Treasures are the Gold Egg, Silver Egg, Crystal Egg, Ruby Egg, Sapphire Egg, Emerald Egg, and the Prism Key. In-game, when the player tries to use any of them, a message is displayed, telling that they might be useful in Prism (but not in Brown). Besides their supposed importance, they can be sold or tossed.
"It's a treasure hunt that people are supposed to take up on their own. If people were just given the locations of these items, the effort taken to find them will not be what Mr. Koolboyman intented. I feel that he is right in this regard, but it is insanely difficult. So my method to revealing the locations of these special items will still keep them hidden but it should vaguely point others in the direction as where to find them."
"Gold Egg: We have escaped the dungeon!"
"Silver Egg: What is this sticky film on the mountain?"
"Crystal Egg: There are no Spinarak in this cave!!"
"Ruby Egg: I hope my game doesn't get deleted."
"Sapphire Egg: A small token on the road to victory."
"Emerald Egg: Pack your bug spray."
"Prism Key: Not all things are reflected in the mirror."