A vajra is a mythical weapon use in many Asian rituals to symbolize a diamond (for it indestructibility) as well as a thunderbolt (for its irresistible force); the Sanskrit word has both these meanings.
The vajra is a club with a ribbed spherical head. It is the weapon of Indian Vedic deity of rain and thunder, Indra. It's symbolism can be found in the dharma traditions of Buddhism, Jainism and Hinduism, often to represent firmness of spirit and spiritual power. In Indian mythology, vajra is one of the most powerful weapons in the universe.
In Buddhism the vajra is the symbol of Vajrayana, one of the three major branches of Buddhism. Vajrayana is translated as 'Thunderbolt Way' or 'Diamond Way' and can imply the thunderbolt experience of Buddhist enlightenment or bodhi. It also implies indestructibility, just as diamonds are harder than other gemstones.