The Holy Diary is a C Tier Rarity item according to the Stand Tier List with a 1/10 chance of spawning every 15 minutes. Or can be bought in the shop for 1,500,000 cash.
The Holy Diary used to have 10 uses, but since then 6 of them have become unobtainable (the seventh being a holiday event). Its first use is turning Star Platinum and The World to make Star Platinum OVA and The World OVA respectively. It can also be used on The World OVA to create Futuristic Shadow The World and finally used on Samurai to make Project Samurai.
The seven Unobtainable stands are
- Gold Experience -> Cranberry Experience(deleted)
- Golden Experience Requiem -> Cranberry Experience Requiem(deleted)
- Anubis-> Phantom Blade(Unobtainble)
- Galaxy Annihilator-> Holy Angelic Platinum(Unobtainable)
- Tusk Act 4-> Shiny Tusk Act 4(Deleted)
- Shadow The World Requiem->Halloween Shadow The World Requiem (Temp Unob for now since it's a seasonal unob it will come back every year).
Aside from these uses, the Holy Diary is also mainly sold for 300,000 Money at the Black Market.
You can also buy it for 1.5M Money at Akira's Bizarre Shop.