We know that a Horcrux is the object in which a piece of the Horcruxee's Soul is stored in.
But these objects were once normal (albeit special) objects before being turned into a Horcrux, and could be destroyed like any other objects.
My question is: what is it about becoming a Horcrux that makes the object (almost) indestructable?
Is it that a powerful enchantment is placed on the object upon Horcruxation? Or something else?
My thinking is that it's not so much the object, but the piece of soul which is difficult to destroy. As Hermione puts it:
"Because a Horcrux is the complete opposite of a human being.”
Seeing that Harry and Ron looked thoroughly confused, Hermione hurried on, “Look, if I picked up a sword right now, Ron, and ran you through with it, I wouldn’t damage your soul at all.”
“Which would be a real comfort to me, I’m sure,” said Ron. Harry laughed.
“It should be, actually! But my point is that whatever happens to your body, your soul will survive, untouched,” said Hermione. “But it’s the other way round with a Horcrux. The fragment of soul inside it depends on its container, its enchanted body, for survival. It can’t exist without it.”
-Harry Potter and the Deathly Hallows - the Ghoul in Pyjamas
So in a way, the object is linked to the piece of Soul's survival.