I've just read an article that claims the following.
When Luke holes himself inside his home, fans get a glimpse of his surroundings where they see a makeshift necklace hanging from the wall. The necklace appears to be a section of a destroyed lightsaber, with a red crystal fixed at the center.
Based on this appearance, it looks as if Luke has kept a portion Darth Vader's destroyed lightsaber as a keepsake, as a reminder of his fateful confrontation with his father that changed the fate of the galaxy.
'Star Wars: The Last Jedi': Was That SPOILER In Luke's Hut on Ahch-To?
I've only seen the film once and I'm not that eagle eyed so missed what was inside of Luke's hut.
Was there a necklace that contained a portion of Darth Vader's lightsaber?