First of all I'm not sure weather I'm correct, but while reading one of English translation of Shiva Puran, I found the translated english of Vajra, which means 'Hard as Lightning'.
This can be metaphoric as Vajra and characteristics of lighning, are similar, whatever thing is thrown to lightning(due to its excessive charge density) either exhausts(or repelled) or is passed by depending upon the 'Mass density : Charge Density'.
In short any thing thrown towards Vajra will be reflected away (repelled), or will be of no use (exhausted) when coming in touch of Vajra. If in some case its more powerful than Vajra, will just pass away through it making the Vajra Practically Invincible.
And due to its High Charge Density Characteristics, can become the cause of excessive lightning.