It's the two-way mirror that Sirius gave Harry two years ago. Pity Harry never thought to use it; he might have saved Sirius's life. It's broken now, and it wouldn't work without a second mirror besides - wait a minute! Did you see that? Somebody's eye was looking back at you... No, must have been a trick of the light. Still, you should keep the mirror shard, and let the trio know about your find. Maybe they know what happened to the second mirror..