Question: why was the sword of gryffindor able to destroy horcruxes?
Answer: Magical properties As Goblin's silver, it imbibes only substances which strengthens it, which can then be used against enemies. This means that it never requires cleaning. Due to Harry Potter killing Salazar Slytherin's Basilisk with it, the sword is imbued with Basilisk venom and can destroy Horcruxes.