"Horace formed a kind of club of his favorites with himself at the center, making introductions, forging useful contacts between members, and always reaping some kind of benefit in return, whether a free box of his favorite crystallized pineapple or the chance to recommend the next junior member of the Goblin liaison Office."
-- Albus Dumbledore (HBP4)
"I confidently expect you to rise to Minister of Magic within twenty years. Fifteen, if you keep sending me pineapple, I have excellent contacts at the Ministry."
-- Horace Slughorn to Tom Riddle (HBP23)
Crystallised pineapple is Horace Slughorn’s favorite sugary treat sometimes received as a gift from his collection of “favorite” people (HBP4).
Once Harry saw Slughorn buying it for himself at Honeydukes in Hogsmeade (HBP12). Crystallized pineapple symbolized his life-long love of luxury, even when Harry viewed him as a much-younger man and Tom Riddle's teacher: "sitting ...in the comfortable winged armchair in his office, his feet resting upon a velvet pouffe, a small glass of wine in one hand, the other rummaging in a box of crystallized pineapple" (HBP23).
Pineapples are traditional symbols of wealth and luck.