Did you read chapter 93 yet? If not, go and read it before you see anymore spoilers. Otherwise, recall that Dumbledore used that magic-detector thing to check Harry's person and all objects in Harry's possession for if they were transfigured from Hermione's remains. Several things turned up magical, and those like Harry's emergency portkey were checked to see if they were transfigured from something else. When Dumbledore got to the ring, that detected as magic (of course) and Dumbledore suspected that rather than the diamond being his father's rock, Harry had secretly transfigured Hermione's remains into the diamond. But when Dumbledore dispelled the transfiguration on the diamond, it was the diamond after all. Thus, Dumbledore's check checked out, and Dumbledore concluded that Harry was not hiding Hermione's remains.