Unicorn Horn

Unicorn horn is an expensive and powerful potion ingredient (PS5, PS15, FB). A unicorn grows its horn at about four years of age (GF10).

The summer before Harry's first year at Hogwarts, silver unicorn horns were being sold at the apothecary in Diagon Alley for 21 Galleons each - three times the cost of Harry's wand (PS5). Later the same year, Harry told Firenze that the first-years had used unicorn horn in Potions class (PS15).

A unicorn losing its horn is mentioned in the song sung by the goblin singer at the Blind Pig (WFT).