The term vajra is a Sanskrit word that is usually defined as “diamond” or “thunderbolt.” It also defines a kind of battle club that achieved its name through its reputation for hardness and invincibility. A diamond is spotlessly pure and indestructible. The Sanskrit word means “unbreakable or impregnable, being durable and eternal”. As such, the word vajra sometimes signifies the lighting-bolt power of enlightenment and the absolute, indestructible reality of emptiness.
The vajra objects usually are made of bronze, vary in size, and have three, five or nine spokes that usually close at each end in a lotus shape. The number of spokes and the way they meet at the ends have numerous symbolic meanings. The vajra often is used together with a bell (ghanta). The vajra is held in the left hand and represents the male principle – upaya – referring to action or means. The bell is held in the right hand and represents the female principle – prajna – referring to wisdom.
At the center of the vajra is a small flattened sphere which is said to represent the underlying nature of the universe. It is sealed by the syllable hum (hung), representing freedom from karma, conceptual thought, and the groundlessness of all dharmas. Outward from the sphere, there are three rings on each side, which symbolize the three-fold bliss of Buddha nature. The next symbol found on the vajra as we progress outward are two lotus flowers, representing Samsara (the endless cycle of suffering) and Nirvana (release from Samsara). The outer prongs emerge from symbols of Makaras, sea monsters.
The number of prongs and whether they have closed or open tines is variable, with different forms having different symbolic meanings. The most common form is the five-pronged vajra, with four outer prongs and one central prong. These may be considered to represent the five elements, the five poisons, and the five wisdoms. The tip of the central prong is often shaped like a tapering pyramid.
(Information from https://www.thoughtco.com)