Gambopa (1079-1153AD) writes the following about vajra samadhi, as translated by Herbert Guenther (and edited here):
"This state of absorption is considered to be like a Vajra, because it is indestructible, strong and stable. . .'Indestructible' means that it cannot be destroyed by impurities; 'strong' means that it cannot be weakened by worldliness; and 'stable' means that it cannot be shaken by discursiveness."