A Vulcan gong
The Vulcan gong was an ancient Vulcan musical instrument.
It was hexagonal in shape and made of an unknown metal. It was struck with a hammer that was partially made from a green stone.
During a Vulcan wedding ceremony, it was customary for the groom to call the wedding party by striking the Vulcan gong. (TOS: "Amok Time")
A similar gong was rung during Spock's fal-tor-pan ceremony in 2285. (Star Trek III: The Search for Spock)