Ron contributed some "crooked tricks" he had heard from the twins: breathing on it, drawing symbols on it with the frosting from Cauldron Cakes, lightly scratching its spine with a quill feather...
The result was nothing but a sweet, cloying smell and a few crumbs on the diary.
Harry tried writing in it directly. Using varying pressure and different inks—even borrowing Hermione's color-changing ink—he wrote all sorts of questions on the blank pages:
Who are you?
Who do you belong to?
Why were you with Ginny?
Why did Malfoy know about you?
Are you connected to the Chamber of Secrets?
Something of Voldemort's?
Speak!
The more he wrote, the more agitated he became. His handwriting went from neat to messy, until he was practically gouging the paper, as though he wanted to carve his anger and questions into it. Yet every word stubbornly remained on the page like stagnant water, without the slightest response.
The diary remained as silent as an icy tombstone.
"Completely useless!" Hermione finally put down her wand in frustration and rubbed her throbbing temples. "Either it has an incredibly sophisticated protective spell on it that we can't understand, or... it needs some specific trigger we don't know about."
Ron yawned and irritably scratched his red hair. "Maybe it's just a particularly sturdy blank notebook? Malfoy was probably just trying to disgust you, Harry. I think we should forget it for now. Just keep it safe and don't give him a chance to snatch it back or report us."
"Ron's right." Hermione stood, deep concern written across her face. "Harry, put it somewhere safe—somewhere very safe. Until we figure this out, don't try anything else so casually. Especially... don't 'talk' to it, if it can talk at all. I have a very bad feeling about this thing."