I'm in the minority here, but I don't see any harm in OP letting Jade know why Ruby is upset. I don't see it as "being caught in the middle" or "it's none of my business" as much as it is helping two of my friends resolve a communication issue.
I was in a similar situation once many years ago, although it wasn't a friendship-killer. I was in charge of organizing an event -- a job that usually fell to someone else but this time it fell to me. Dave likes to do X at this sort of event, but he only does it when the person who usually does X isn't available. I told Dave a couple of weeks before the event that I would like to do X this time, *if it was OK with him.* He misheard this as "I'm going to do X." And he bailed on the event, to punish me for appropriating his X.
If it wasn't for a mutual friend telling me *why* Dave bailed, and then telling Dave how upset I was that he'd misunderstood me, Dave and I would not be friends today. As it turned out, after this mutual-friend-facilitated communication, Dave contacted me, explained his position, we exchanged apologies for the misunderstanding, and we made up.
If Jade were given a chance to explain and apologize, I think the friendship could be saved, or at least ended amicably. But if nobody (Ruby or OP) is willing to let her know exactly *what* she did wrong, that can't happen.
I'm not inside Jade's brain, so I have no way of knowing what the deal was. Did she forget her promise to Ruby? Did she flake on telling Diamond about the plan? Did her email access go south while she was out of the country? I'm not trying to make light of Ruby's disappointment in her, but I would like to think if she actually *knew* the reason for the cut, she would be falling all over herself to make amends.