Vajra  is a Sanskrit word meaning both thunderbolt and diamond. As a material device, the vajra is a ritual object, a short metal weapon - originally a kind of fist-iron like Japanese yawara - that has the symbolic nature of a diamond (it can cut any substance but not be cut itself) and that of the thunderbolt (irresistible force).

Vadzrapani holds one.Notice the lasso it represents the shen-rope:

And this is Vajrapani gandhara (on the right) holding a flywhisk:

These flywhisks are very sacred as they represent the thunderbolts:

Here the flywhisk in ritual procession in egypt: