Possession is the least of the problem here, but we'll deal with that first.
Possession has two relevant meanings: to have immediate physical control of something, or to own something. However, the phrase "in the possession of" always exclusively uses the immediate physical control meaning.
The Hope Diamond lying in its security case isn't in the possession of anyone, so that's the first hurdle of four cleared.
Second hurdle: spell range is Touch
The Hope Diamond in its security case cannot be the target of Swift Ready because the spell's range is Touch, and the whole point of the security case is preventing anyone from being able to touch the Hope Diamond. Trying to case the spell would have failed for lack of a legal target.
Third hurdle: the Hope Diamond never qualifies as a legal target for Swift Ready
Swift Ready is very specific about what it can target:
- One suit of armor or other item of clothing (such as a robe or cloak); plus either
- One or two weapons, tools, or magic items, each small enough to be wielded in a single hand; or
- One weapon, tool, or magic item that is normally wielded in both hands.
The Hope Diamond is not:
- A suit of armor or other item of clothing
- Any kind of weapon, tool, or magic item (regardless of size or number of hands required)
Therefore, even if the caster can touch the Hope Diamond, the spell would fizzle for lack of a legal target.
Fourth hurdle: the spell effect requires line of sight
The main effect—teleporting the target to you later—requires that you have line of sight at the time of the teleport. If it wasn't for the above hurdles, that would be useful for getting something out of its security case, but since those are factors, this requirement means it has very limited utility.
Due to this requirement, all the spell's effect does it move something from over there to over here, under conditions where you could just do that without the spell anyway. It just does it instantly instead of having to walk over and pick it up.
What Swift Ready is for
Swift Ready is just a way to quickly equip your own equipment. It's not intended and can't be used to enable untraceable theft.
You could maybe pull off this scheme with an unprotected item of valuable equipment left open to the public with long sight-lines, but that's a pretty limited kind of caper that a thief won't be able to pull off repeatedly, simply for lack of valuable targets that are vulnerable in these precise ways.
I agree though, that particular situation sounds like it was a cool moment. You don't necessarily have to retcon that, and I wouldn't suggest you do.
However, have a talk with your player about the correct use of this spell. Right now your player is gleefully thinking up how to rob the entire world blind using a pedestrian 2nd-level spell. The sooner you disabuse them of this idea, the healthier will be your game.
Explain away the successful theft of the Hope Diamond however you like (maybe an unseen benefactor helped?!), but say in no uncertain terms that this trick was a one-time use.
Or ride that spell into the ground!
Alternatively, retcon the spell to work exactly like it did, and enjoy the hard left turn into a riotous, high-flying "D&D: The Stealening" campaign. Hold onto your hat (else it be stolen) and brace yourself for the biggest Monty Haul campaign ever!