Berry Juice is a HP restoring item introduced in Generation II. It is produced by giving a Berry (in Generation III an Oran Berry) to a Shuckle to hold. According to Shuckle's Pokédex entries, the berries it stores in its vase-like shell decompose and become a gooey liquid that is Berry Juice. Berry Juice restores 20 HP.
In Generation II If the Berry Juice is held by a Shuckle, it will, over time, harden into a Rare Candy. This has been removed for use in later games.
Locations
Omega Ruby and Alpha Sapphire: This can be bought in Mauville City.
Trivia
Berry Juice is one of few edible items known to be produced by a Pokémon. The others are MooMoo Milk (which is produced by Miltank) and Honey (which is produced by Combee).
The occurrence of these Pokémon holding these items when found in the wild is also 100% in all three cases.