Orb of Chance lets you upgrade a white item into an item of random quality, including very rare Unique items. There are a few items that you should always pick up and use one Orb of Chance on, because they have a high chance to give you a very valuable Unique.
Getting the Unique item is extremely rare, but if you ever get it, you will get an extreme amount of currency, so this gamble usually pays off once you get your first good Unique item from doing it.
Here are all the items you should use Orb of Chance on:
Sorcerer's Boosts -> Skyforth.
Occultist's Vestment -> Shavronne's Wrappings
Glorious Plate -> Kaom's Heart
Assassin's Garb -> Cospri's Will
Jewelled Foil -> Cospris's Malice
Note that the name on the left is the white items you want to pick up, and the same on the right are the Unique items you want to get by chancing them.
The Sorcerer's Boots are the ones that are worth the most if you get Skyforth, so if you only have a limited amount of Orbs of Chance, use them on these boots!