Gavel

A gavel is a hammer-like tool used by judges and arbiters to demonstrate their power and authority in a formal trial or hearing. Gavels were used by Humans, Klingons and Bajorans.

The Klingons used a gavel with a large, outstretched arm, and their version of the tool was a metal ball. Courtrooms located on Qo'noS and Narendra III from the 22nd century to at least the 23rd century had this type of gavel. Klingon gavels emit sparks upon hitting. 

In the Cardassian judicial system, a symbolic rock was used (such as by Chief Archon Mok'bara on Cardassia Prime in 2370) in place of a gavel. 

In 2152, a Klingon magistrate used a gavel during Captain Jonathan Archer's tribunal on Narendra III. 

Adam – one of multiple disciples of Dr. Sevrin, who were brought aboard the USS Enterprise (NCC-1701) in 2269 – sang a song that contained the word "gavel" upon Captain James T. Kirk refusing to release Sevrin from medical confinement (since he was a carrier of synthococcus novae). The lyrics Adam sang were, "Stiff man putting my mind in jail. And the judge bang the gavel and say, 'No bail.' Gonna lick his hand and wag my tail."

Captain Kirk had a more concrete encounter with a gavel in 2293 as, in that year, one was used by an albino Klingon speaker, one of three hooded judges at Kirk and Dr. McCoy's trial on Qo'noS. 

In 2369, the Bajoran arbiter Els Renora used a ball-gavel during the hearing of extradition for Lieutenant Jadzia Dax on Deep Space 9. 

Background
In the script for Star Trek VI, the judges' gavel in that film is described as "a grotesque mailed glove," mistaking the gavel for the glove that holds it. 

A concept drawing for the Star Trek VI gavel was created by Nilo Rodis-Jamero. (The Art of Star Trek, p. 267) Props master Gregory Jein carved the ball shape out of green foam, then had one of his employees in the props department cast the actual ball. Jein was aware of deviating from the red paint scheme commonly associated with the Klingons, opting to give the gavel a weathered steel finish. The prop was made so that streaks of light would burst out of the cracks in the gavel as it was struck. However, there was a difficulty with the gavel's interior light during filming. "The actor got too enthusiastic and the light broke on the second or third take," said Jein. (The Making of the Trek Films, 3rd ed., p. 128)

The gavel glove from Star Trek VI was made out of molded fiberglass, weighed about ten pounds and could actually be worn as a massive glove. ("Penny's Toy Box", Star Trek VI: The Undiscovered Country (Special Edition) special features) Since the actor playing the Klingon judge (Robert Easton) had arthritis, the glove had to be customized for the performer by making it softer to wear. Greg Jein remarked, "We had to hollow half of it out and pad it up." (The Making of the Trek Films, 3rd ed., p. 128)

For the gavel glove in Enterprise, the production team obtained the same model as was used in Star Trek VI, then made a cast from it. "Judgment" writer David A. Goodman was not entirely certain whether the sparks from the gavel were added in post-production or involved live pyrotechnics but reckoned that at least some were probably added after-the-fact, remarking, "I don't know that there's any way they could have done that live." ("Judgment" podcast/audio commentary)

The Bajoran gavel seen in "Dax" was sketched by Rick Sternbach and made out of real stone by the art department. (DS9 Season 1 DVD special feature "Deep Space Nine Sketchbook")

A gavel was scripted to appear in, to be used by Data while acting as arbiter of a trial to determine whether an apparently non-corporeal lifeform was Ardra, but no gavel appears in that episode's final version. Similarly, the script for "Tribunal" mentions a gavel though none is technically shown in that installment either, with the episode's teleplay specifying that Mok'bara uses "not a gavel but a symbolic rock."