The USS Kongo is named for Mount Kongō (Japanese: 金剛山 "Kongô-zan"), a mountain not far south of the old capital of Japan at Nara. The word kongo was the Japanese pronunciation of the Chinese translation of vajra in Sanskrit.
The vajra was originally the thunderbolt of the god Indra. Later in esoteric, Tantric, or vajrayona Buddhism, the vajra symbolized supernatural powers that could be obtained through esoteric rituals. In the sexual symbolism of Tantrism, the vajra was also associated with the male organ. Kongo can also mean a jewel or a diamond, or hard as a diamond. So appropriately Kongo as the name of a ship would imply the great power of a thunderbolt or the hardness of a diamond.