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We have heard about the Vajra holded by Dev Indra, which was said to be from the bones of Rishi Dadhichi, but before that, while saving Devi Sati from a rakshas Rishi Dadhichi earned a boon from Lord Shiva, that his bones will become of Vajra.
What does this Vajra mean?
To answer on etymological roots of Vajra as OP asks in the comment (I persume he mistyped entomology which is used in context of insects ):
The word Vajra (वज्र) comes from the root Vaj (वज्) which means to feather an arrow.
The word Vajra means:
hard, severe, forked, zigzag.
a thunderbolt (said to have been formed out of the bones of the sage Dadhichi) as referenced in the Sloka: आशंस्ते समितिषु सुराः सक्तवैरा हि दैत्यैरस्याधिज्ये धनुषि विजयं पौरुहूते च वज्रे
any weapon like a thunderbolt
a diamond - वज्रादपि कठोराणि मुंदूनि कुसुमादपि
an instrument for performing jewels - मणौ वज्रसमुत्कीर्णे सुन्नस्येवास्ति मे गतिः
Steel
a kind of talc
Taken from The Practical Sanskrit-English Dictionary by Vaman Shivaram Apte.
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.
Once a demon Vithrasur was trying to kill Goddess Sati before her marriage with Lord Shiva. Sage Dadhichi was trying to save Sati from Vithrasur. Sati was very worried. Vithrasur was hitting Dadhichi with his powers. Dadhichi was injured. Then Lord Shiva had saved Sati from Vithrasur. Lord Shiva had blessed Dadhichi that his bones will become a weapon. One day in the time of Goddess Parvati,Lord Shiva had requested Dadhichi to donate his body for killing Vithrasur. Dadhichi had donated his body and he had died. Lord Indra had made a weapon with Dadhichi's bones. Shiva had named the weapon Vajra. Then Indra had killed Vithrasur with his Vajra. So,Vajra was a weapon of Lord Indra which was made with Sage Dadhichi's bones after his death.
