They wouldn't have refused to cover you. What they probably said was that a safe warranty would apply to anything over 10k. Meaning when the item of jewellery is not being worn it must be kept in a locked safe. Most insurers apply this warranty.
And it was likely that 123.ie would have applied the same warranty. Was the premium for last year without the ring. And now this year its 60% up with the ring? If so seems perfectly fair to me. What else would you expect? If your premium for the year was €120 for just a contents policy, you would probably be looking at an extra €225 for the ring alone. Most specified items are rated at about 1.5%.