I’m no one to dispute QT on his own movie, but Vincent says to his drug dealer that someone keyed his car “the other day.” Most people, even while buying smack, can keep “the other day” straight from “a few hours ago.”
Bumped.
The answer is obvious: Redirecting...
It’s radioactive material, but better-contained than when Mike Hammer dealt with it:
The finale of Kiss Me Deadly
One of the things that fit all the criteria is the Holy Grail.
- Kids held it back from Marcus because he couldn’t be doing anything good with it. Maybe they intended to give it back to the museum or wherever it came from. My theory is that it was being heisted, and the thieves were killed transporting it. The kids just happened to be near and grabbed it. They were trying to offload it when word got around to Marcus that they had it. Someone contacted the kids, and they wouldn’t give it up, not realizing how much shit they were in, but one knew who Marcus was and scared to death, told on them.
- Light enough to carry it around, the inside of the briefcase had padding so it wouldn’t roll around and make noise.
- “Is that what I think it is?” - Ringo wouldn’t have seen it before, but would have an idea just looking at it. A golden mug glowing brightly, not that many of them around so…
- “It’s beautiful” - priceless genuine brightly glowing holy object, I’d be in awe of it too.
- “The bosses dirty laundry” - sarcasm, but true without making someone tempted to see it or snatch it themselves.
It’s the only thing I can think of that fits all the criteria
Sorry, I just woke up. 
Forgot to add:
- Kid using a gun to protect it, but either was extremely nervous or the grail protected Vincent and Julius.
- Julius’ religious experience and talking about quitting his job in the diner.
Macguffins, from a new favourite Youtube channel.