Mymble's Diamond

Mymble has found a ring on the ground with a big, sparkling stone and gives it in to the Inspector. When no one comes forth to claim it he gives it to her and she is very, very happy, deciding it must be a diamond. She parades it all around Moominvalley, until she is informed by Fillyjonk that if one has a huge diamond like that one must dress accordingly.

Synopsis excerpt via Kitsu.