Snoke's ring = a lightsaber crystal?
I'm going to preface this by saying that Disney's current "all Star Wars media we crank out is canon" policy extends to their theme parks as well. Matt Martin himself confirmed that the story you experience in the newly minted Galaxy's Edge attraction at Disneyland is meant to be canon as well...
"The events at Galaxy's Edge are meant to be canonical," Martin explained. "I wouldn't say that you as an individual, dressed in Earth clothes, are now part of actual canon, but the storytelling is meant to really be part of Star Wars, down to its place on the timeline. I can't get into it, but I know the specific day that the events of this park take place."
So on that note, it seems like the attraction has subsequently confirmed the 'importance' of the ring Snoke was shown wearing during TLJ:
The Visual Guide stated that it was a piece of obsidian from beneath Vader's Castle:
That being said, evidently one of the lightsaber crystals you can happen upon in the 'build a lightsaber' part of the attraction is a rare black crystal:
And this message comes with it:
This is NOT a Darksaber crystal. The lightsaber's blade color is still red(which makes sense given that the obsidian is infused with the dark side), and when placed inside one of the Sith Holocrons you can also purchase at the park(lol), it comes with special messages from Snoke: #RandomYouTubeExample
So yeah, it's obviously just another cash-pull from Disney, but that is evidently the 'official' explanation they're giving us for Snoke's ring: it is a lightsaber crystal.