When Dumbledore and Harry visit Horace in HBP, Horace seems to react to the ring that Dumbledore was wearing. Does he recognise this from Tom Riddle wearing it when he asked about Horcruxes or why did he react like that?
"I am undoubtedly slower than I was. But on the other hand..." He shrugged and spread his hands wide, as though to say that age had its compensations, and Harry noticed a ring on his uninjured hand that he had never seen Dumbledore wear before: It was large, rather clumsily made of what looked like gold, and was set with a heavy black stone that had cracked down the middle. Slughorn's eyes lingered for a moment on the ring too, and Harry saw a tiny frown momentarily crease his wide forehead." Half Blood Prince - chpt 4