Instructions
Fly to Violet City using your Pidgey or another Flying-type Pokemon.
Jump on your bicycle and go west until you see a large cave.
Enter the cave and use Pikachu's "Flash" technique to illuminate the darkness.
Walk forward until you see a hole.
Fall into the hole. You will arrive in a new room with another hole. Fall through this to find yourself in the room that contains the Omanyte Puzzle.