Holmes held up the blue carbuncle, cold and brilliant like a star—why did Ryder's face harden between claim and denial?

Our visitor staggered to his feet and clutched the mantelpiece with his right hand. Holmes unlocked his strong-box and held up the blue carbuncle, which shone out like a star, with a cold, brilliant, many-pointed radiance. Ryder stood glaring with a drawn face, uncertain whether to claim or to disown it.