The Diamond Ring has 10 charges to increase your HP, and each invoke makes it lose one charge. Invoke it ten times, and you lose the ring.You can also use it to scratch open the windows in the City of Sky to get all the best items, not just opening one window with the consumable Light Key. Each scratch also causes it to lose a charge.
I've seen some posts of people claiming that the Diamond Ring can only be used to scratch the City of Sky windows once, and I can pretty much guess they simply invoked it one less time than needed to lose the item (9), then used it once later at the City of Sky and lost it, making them erroneously think that they could only scratch with it once.
Basically, invoke it six times on your character(s), save three for the window scratching, and the last charge will keep it around so you can continue to equip it.