Smoky Quartz is a fusion between Steven and Amethyst. They were first formed in Earthlings.
Smoky Quartz has shown up three times, once in Earthlings fighting Jasper, once in Know Your Fusion to show Garnet and Pearl (and Sardonyx) who really fought Jasper, and once to save Peridot from a Corrupted Gem during Back to the Kindergarten.
borrowed from steven-universe.wikia.com
As a fusion, Smoky Quartz adopts both Steven's and Amethyst's personalities. In this case, Smoky Quartz has a proud, carefree, excited, and happy-go-lucky attitude. They are described by Rebecca Sugar as being "a big happy kid with a yo-yo". They also have a humorous and relaxed side to their personality with a tendency to make jokes even in the heat of battle. They restored some confidence to Amethyst, who had become depressed and unmotivated after being poofed at the hands of Jasper and ultimately unneeded when Stevonnie won the battle. Their humor can also be rather self-deprecating at times, as shown in the episode, "Know Your Fusion".
They are extremely confident in battle because of their cooperators' trust in one another, and as a result, they are a moderately stable fusion. However, like most fusions, they can defuse when under stress or intense emotion, as shown upon witnessing the beginning of Jasper's Corruption.
However, as shown in "Know Your Fusion", Smoky also has both of Steven and Amethyst's self-doubts added together. During Sardonyx's attempt to find a more "serious" aspect to their fighting style outside of their yo-yo weapon, Smoky was shown to lose more and more self-confidence in themself as they failed at every test thrown at them. They make statements such as "zero plus zero equals zero" and "two wrongs don't make a right. I guess I'm living proof that's true," even going so far as to call themself "one big super-wrong, good-for-nothing dud with a yo-yo."
Smoky Quartz can summon a yo-yo and do several tricks including spinning it, hitting Gems with it, and turning it into a fan of sorts.
When fused with Jasper, Garnet (Ruby and Sapphire), Pearl, Amethyst, Lapis, and Peridot, they form Black Opal.