Party pet made a rainbow pearl and I need to decide what it is by next session

A while back, my party encountered a Chest weasel, a fey creature made on the Pointy Hat YouTube channel that feeds on frustration. My party frustrate eachother and everyone they meet with their nonsense, so they were able to convince the weasel to come along, promising it would never go hungry. Every time they are particularly frustrating, I've had the overindulged weasel blep up some rainbow vomit.

Flash forward to the weasel limp with a distended stomach, occasionally leaking rainbow liquid from his mouth. I had him disgorge a small, dense rainbow pearl, then feeling better. I came up with it on the spot and have no idea what it should be. Any ideas?