Tyrande, Malfurion, Illidan and the Shadowsongs will have words with you that it only belonged to the Highborne. Highborne led or even highborne rescued doesn't make Suramar highborne owned. It belonged to ALL the night elves of Suramar - those that left and survived were the ones who beat Azshara and the legion and became the Vigil night elves, those that stayed and fought to protect the city became the nightborne - highborne and non-highborne alike.
Saying that, we don't really know if Tyrande came from a highborne family - we just know that she accepted the call to the priesthood which means you sever all former ties - we don't know what those ties are. We know the Shadowsongs aren't, but we're not sure of the Stormrages, I suspect they were not highborne, but I don't remember it explicitly mentioned, Malfurion rejects culture of the day as too complacent and misguided - he has the wisdom to see what all the people were going - it reminds me of our modern society - where you get some wise content souls that notice the misguided way we live - despite all our accomplishemnts most people don't care aobut the right things, have their priorities misplaced etc - and he ofc eventually does something about it in a way that has nothing to do with the culture norms. He finds his calling in nature, and because of his success most night elves after the sundering follow him.
Illidan on the other hand is just as passionate about his people, but goes or tries to go about it via the established route as he loves sorcery, eventually finding out his and his people's level is inadequate to deal with the level of threat the Legion and it's Lord's hold which sets him down his path. The right and wrong of it is not the issue, each must judge, but what is clear is that these brothers have much passion and strength to save their people. I suspect they were not highborne though, or more would have been made of it, but I don't think it was mentioned either.
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yes, that sounds right. So even if you are connected to the Sunwell, you can overload, now most elves would never go there, the sunwell providing the radiance they need, and those that train have the discipline ingrained into them, - no group of elves wanted an Azshara repeat, and the high elves were earnest about making a magical society that worked, they believed it could, Illidan did too, except Malfurion didn't trust it would or at least did trust that they could do it responsible at the time. He is proven to be wrong.
We see the Suramar night elves another example of those who achieved magical usage with discipline and control to their incredible heights, which makes it ironic because the nightwell gets corrupted anyway, but unlike the Shen'drelar or the highborne or the high elves - it's not via reckless abuse or arcane addiction or greed. Another irony too about the nightborne is their betrayal of their people to the Legion, Elisande wasn't corrupted by arcane magic, or in a reckless spree displaying the callous arrogance and hubris of Azshara... but she still gives in to the Legion. This is the outstanding arcane night elven group, and without magical induced madness, some of them do anyway. Some don't also, so...
Back to the gem. I'm now imagining as a thing they open up and then just sucks them in, even when they try to stop absorbing the arcane they can't, like it's spelled so they have no choice but to overload (my imagination)- we never get to the bottom of who does this to them.