Silver and Opals
Chapter 12 of Harry Potter and the Half-Blood Prince: Silver and Opals
Despite telling Harry how important the lessons are, Professor Dumbledore is often absent, and there are no additional lessons before the first Hogsmeade weekend in mid-October. While reading the Half-Blood Prince's textbook one morning, Harry casts a non-verbal one, Levicorpus, and unintentionally levitates a sleeping Ron. Harry frantically invokes the counter spell.
Hermione is unamused when told about Ron's experience. This Prince could be a dodgy character, and Harry is invoking his spells without knowing their effect. Harry recognizes the spell from Snape's memory in the Pensieve as the same one his father used on Snape. Harry briefly considers that his father was the Half-Blood Prince, but discounts this because James was a pure-blood wizard. Meanwhile, Ginny delivers a note to Harry from Dumbledore, scheduling another lesson for Monday evening. Ginny mentions she is going to Hogsmeade with Dean and may see them there.
On Saturday, after being scanned by Filch with Secrecy Sensors, the Trio embark to Hogsmeade. Finding Zonko's joke shop boarded up, they move on to Honeyduke's and run into Professor Slughorn, who extends yet another dinner invitation to Harry and Hermione. Harry is grateful Dumbledore's lesson gives him an excuse to decline, though Ron feels slighted.
On the way to The Three Broomsticks, Harry sees Mundungus Fletcher talking to the Hog's Head Inn barman. Mundungus drops a suitcase, spilling its contents, including a silver goblet bearing the Black family crest. Harry, enraged, seizes Mundungus' throat, claiming he has looted Sirius Black's house. Mundungus blasts Harry away and Disapparates. Tonks appears and says it is useless to hunt for him, but Harry intends to report Mundungus to Dumbledore.
Hermione suggests heading back to Hogwarts, to which Ron and Harry agree - this has been an unpleasant outing. They follow Katie Bell and her friend Leanne, who are arguing over a package. Leanne tries to take it from Katie, but it rips open. Katie, suddenly deathly still, rises into the air, then falls to the ground, writhing and screaming in pain. Harry runs for help and returns with Hagrid, who carries Katie to the school. Harry prevents Ron from touching the package, recognizing a necklace inside as the same one he saw at Borgin & Burkes in Knockturn Alley that bore a label reading, 'Cursed.' Leanne says Katie returned from the toilet carrying a package that she insisted she must deliver to someone at Hogwarts. Leanne suspects Katie was under the Imperius Curse. Harry immediately suspects Draco is involved, though Ron discounts this.
Professor McGonagall meets them at the gate, and Filch takes the necklace to Professor Snape. Harry suspects Malfoy was involved, but McGonagall says Draco was doing detention with her. Dismissed, Harry, Ron, and Hermione wonder who the necklace was intended for. It seems unlikely that anything harmful could get past Filch and his Secrecy Sensor. They conclude it was a poorly thought-out plan.
The attack on Katie Bell bolsters Harry's suspicions that Draco is directly involved in some sinister plot, though there is only circumstantial evidence linking Draco to the necklace, and he has an iron-clad alibi. Regardless, Harry remains positive that Draco was behind the attack. While readers can sympathize with Harry, knowing that Draco has been tasked with some unknown mission for Voldemort and that it probably is linked to this incident, Harry lacks any objectivity whatsoever. He becomes so single-mindedly convinced that Draco is guilty that he stubbornly refuses to consider other possibilities. Also, the Trio's belief that the attack seemed poorly planned may be more significant than they realize.
Adding to Harry's concerns is Dumbledore's increasingly frequent and unexplained absences. Not only is this disturbing to Harry, but probably the entire school. Dumbledore is Hogwarts' symbol of authority and security, and his empty chair in the Great Hall may signal that there is greater turmoil in the wizarding world than anyone realized. Harry, in particular, is affected by his absence. Harry still deeply mourns his godfather Sirius' death, and Dumbledore has become even more a father figure to him. When a parent is away, a child often feels abandoned and unprotected, even when left in capable hands.
Harry physically attacking Mundungus shows a rare side to his personality. While Harry is often quick to anger, he normally maintains control and never reacts physically. Still grieving his godfather's death, he is so outraged that Sirius' possessions (which now belong to Harry) have been violated that he is overcome by rage. Only an extreme circumstance could have provoked such an outburst. And only Tonks' intervention prevents Harry from inflicting serious harm on Mundungus and possibly facing severe legal consequences.
Harry's hope that the clever Half-Blood Prince could actually be his father may indicate he is looking for even more redeeming qualities in James, though he is quickly forced to discount his own theory. Harry may still have conflicted feelings over his father. Although he loves the man he never knew and knows he was a good and brave person, he was deeply disappointed when he accidentally learned (in Harry Potter and the Order of the Phoenix) that the youthful James was once a rather obnoxious bully. James, along with Sirius Black, often tormented and humiliated Severus Snape purely for their own amusement while they were students at Hogwarts, though James outgrew his childish behavior.
- Who does Harry think might be the Half-Blood Prince? Why does he want to believe this and what finally makes him discount it?
- Who does Harry attack and why?
- Where has Harry seen the necklace before, and who does he connect with it? What does McGonagall have to say?
Further Study 
- Why would Harry use an unknown, and potentially lethal, spell on Ron without knowing its effects? What does Hermione have to say? Is she right?
- What might account for Dumbledore's frequent absences? How does it affect Harry?
- Who might have given Katie the package? Why?
- Why was Katie Bell (unknowingly) chosen to smuggle the necklace into Hogwarts if Filch was likely to detect it with his sensors? Was it a poorly though-out plan as Harry, Ron, and Hermione believe it was?
Greater Picture 
Despite Hermione's warning, Harry will continue using the Prince's spells without any regard to their function. Although the one he used on Ron resulted in a humorous, and generally harmless, episode, a curse Harry later casts on Draco Malfoy in anger will have a more devastating effect. Snape, recognizing the curse, intervenes in time to save Draco's life.
Katie Bell was put under the Imperius curse, not by Draco, but by one of his agents, Madam Rosmerta, who in turn had been placed under that curse by Draco. This is the first of three failed attempts Draco makes on Dumbledore's life. However, the plan may not be as ill-conceived as the Trio believes. In particular, we discover later that Secrecy Sensors detect secrets; however, bottling a love potion in a perfume bottle is sufficient to allow it to pass both visual scrutiny and the Sensors. As Katie Bell unknowingly was carrying something lethal, had no malign intent, she was therefore not intentionally concealing any secrets, likely enabling her to pass through the Secrecy Sensors undetected. A poisoned mead bottle also makes its way into the school without detection, and the Trio believes it may have entered unscanned, with one of the teachers.
When Harry encounters Mundungus Fletcher in Hogsmeade, Aberforth Dumbledore, the Hog's Head Inn barman, has apparently just refused to buy something and walked away. It is clear that, having ransacked Sirius/Harry's house, Mundungus is attempting to sell his spoils when Harry confronts him. It is learned later that Mundungus sold the mate to Harry's broken two-way mirror to Aberforth, who was told the mirror's function by his brother, Albus. Late in the final book, Aberforth admits he periodically used the mirror to check on Harry. Harry will occasionally catch Aberforth's eye in the mirror, mistaking it for Albus Dumbledore's, which is apparently similar in shape and color.
Another item that Mundungus loots from Grimmauld Place will play an important role in the final book. When Grimmauld Place was being cleaned, "a locket which none of them could open" was found. This locket will prove quite valuable to Harry, but he will have to trace it through a long chain of thieves before reclaiming it. Kreacher had retrieved it from the trash, and by now Mundungus has stolen it from Kreacher. At some point, it is extorted from Mundungus, and it is from this final thief, Dolores Umbridge, that Harry will have to collect it.