Dorje (the Tibetan name) are also known as Vajra, which is a Sanskrit word meaning both thunderbolt and diamond.
It is a weapon which is used as a ritual object to symbolize both the properties of a diamond (indestructibility) and a thunderbolt (irresistible force).
The Vajra is essentially a type of club with a ribbed spherical head. The ribs may meet in a ball-shaped top, or they may be separate and end in sharp points with which to stab.
The Vajra is used symbolically by the dharma traditions of Buddhism, Jainism and Hinduism, often to represent firmness of spirit and spiritual power.
The use of the Vajra as a symbolic and ritual tool spread from India along with Indian religion and culture to other parts of Asia.