"But," she exclaimed, suddenly, "arsenic is indelible, indestructible; in whatsoever way it is absorbed, it will be found again in the body of the victim from the moment when it has been taken in sufficient quantity to cause death."
"Yes, yes," repeated Villefort, as he approached his home—"yes, that woman must live; she must repent, and educate my son, the sole survivor, with the exception of the indestructible old man, of the wreck of my house.
There are no more uses of "indestructible" in the book.
Show samples from other sources
When the fear takes over, use the only thing more powerful, more indestructible, to fight it: your spirit.
Sabaa Tahir -- An Ember in the Ashes
"Nick, don’t let one good turn make you feel indestructible," Go said.