I would leave the diamond ring at home in a safe or the safety deposit box at your bank.
If it's a valuable piece, the purser's office has safety deposit boxes on most ships ... they are far more secure than in-cabin safes.
Just a hint ... the people with the most big jewelry leave the goodies at home. There is no need to impress people whom you will never see again. And the folks who are NOT wearing big jewelry recognize each other.
We've always kept our travel documents, passports, cash in the safe while cruising and have never had a problem. So far the safes we've had you make up your own combination to open them, so its as safe as the number, no need to worry. I would also follow Pam's advise and keep the diamond at home, there are too many other chances of loosing it or getting lost while traveling, I never take any of my good stuff :-)
Room safes are as safe as you will find travelling. My suggestion though is to catch Diamonique week on QVC. Invest about $100
and you'll have a fabulous looking wardrobe of jewelry and no cares or worries about the real thing getting lost. This is what my wife has done recently. She gets as many complements on the fake stuff as she used to with the real stuff and is much more comfortable about traveling with it.
My wife insists on taking along expensive jewelry contending "what have I got it for ?" I was employed many years in the insurance industry and underwrote jewelry for a number of insurers including
Underwriters At Lloyds. If you cannot leave expensive jewelry home and have it insured make sure your insurer will accept the safe in the stateroom ,IF the policy contains a safe warrnty that requires storing in a safe when not worn. If the insured will accept the stateroom safe, get it in writing.
The cruise ship safe at the front desk is not always the answer since they limit their liability although some will sell you a short term policy for a rather high premium. Keep in mind under this scheme the insurer has no idea as to the value of your jewelry so even in case of a claim there could be a difference of opinion as to the value of the items. Our expensive pieces of jewelry all are insured on the basis of a a current appraisal. The insurer's policy does not have a safe or hotel, motel warranty. Hope I have been helpful.
I like the suggestion the fake stuff that sparkles attracts just as much attention from the wrong people. My wife's diamonds go into the safe deposit box at the bank when we cruise (except for the Raddison Mariner) but she does take a collection of crystal or whatever it is.
Felt comfortable keeping passports in the room safe. Last cruise used room key credit card to open and to close. You had to use the card that last closed the damn thing to open it the next time, which I would remember when my wife was at the other end of the ship.