How does Papa make Billy feel better about the glitter Sal poured on his Diorama in the novel, The Year of Billy Miller?

Papa is able to remove most of the glitter, but he goes onto explain to Billy that in real life, rocks will sometimes sparker because some are composed of mica, a shiny mineral. Mrs. Silver contends that the glitter in Billy’s diorama is a very good effect. Sal is forgiven.