The Infinity Gems are some of the most powerful artifacts in the entire Marvel universe. The six powerful artifacts represent the fundamental pieces of the universe. Claiming them can give the wielder total control over Space, Time, Reality, the Mind, Power and the Soul; combining all six grants omnipotence The quest for the gems served as the overarching plot of multiple Marvel Comics stories as well as the narrative of the first three phases of the Marvel Cinematic Universe.
However, the aforementioned six aren't the only ones that have ever appeared. Others have shown up for brief appearances in cartoons, video games and comics. Below are the four Infinity Gems that even the most hardcore Marvel fan may not know about.
The Rhythm Gem
Marvel Super Hero Squad was a decidedly and purposefully light-hearted take on the Marvel Universe. A video game adaptation of the cartoon series, Marvel Super Hero Squad: The Infinity Gauntlet offered up a humorous take on the same storyline that would eventually inspire the plot of Infinity War and Endgame. In it, the Avengers had to travel around the universe in pursuit of the various Infinity Gems.
One of them, the Rhythm Gem, was stolen by Loki and taken to Olympus. Thor and Falcon come to claim it and eventually have to come to blows with the Greek god Hercules (who here isn't an Avenger but a dumb henchman to Loki and Enchantress.) The disco ball design of the Stone initially gives the impression of power but is ultimately revealed to be useless. In reality, the Rhythm Gem was a fake created by Loki. This forced two of the Avengers on a wild goose chase, keeping them from searching for the real Gem. This particular Gem has (understandably) not appeared since.
The Build Stone
Introduced in LEGO Marvel Super Heroes - Guardians of the Galaxy: The Thanos Threat, the Builder Stone is the perfect tool if you happen to live in a world made out of something as flexible as LEGO. The main plot of the animated series centers around the Guardians of the Galaxy trying to transport the Stone to the Avengers for safe-keeping. But Ronan, Yondu, Nebula and ultimately Thanos himself aim to steal it from them and use it to build terrible weapons of mass destruction.
The cartoon is decidedly silly (though maybe not as much as Marvel Super Hero Squad), but the concept behind the Builder Stone is interesting. It functions a bit like the Reality Stone, creating new things out of nothing. It's capable of building theoretically anything, which would make it a very bad thing if, say, a genocidal Titan was able to get ahold of it. The stone is specifically used later to create a Death Star-sized weapon for Thanos known as the BLT - Big Laser Thing. The stone is perfect for the constant construction of a LEGO world, and eventually ends up taken to Asgard for protection.
The Death Gem
Like the Rhythm Gem, the Death Gem isn't an actual Infinity Gem, at least not in the strictest sense. The Gem was created in the Secret Wars tie-in miniseries, Infinity Gauntlet by Gerry Duggan and Dustin Weaver. This Gem was a false one, created by Anwen Bakian during the course of the miniseries as the ultimate trap for Thanos. While working with the Titan to find the Infinity Stones, her mother (who is a member of the Nova Corps) is killed and he attacks her family
But Anwen manages to claim the Reality Gem before Thanos, and used it to create the Death Stone. Despite looking like the Reality Stone, it has the power to reduce whoever uses it to nothing more than ash. Thanos finds this out the hard way when Anwen "surrenders" the gem to him so he'll spare her family. Having assembled all of the other gems, Thanos briefly believes he's gained omnipotence. But the stone activates within moments, killing him instantly. The ending of the comic suggests that Anwen then takes the six Infinity Gems and uses them so she can be reunited with her mother.
The Continuity Gem
The Continuity Gem comes into play in only one issue, and for such a 4th wall breaking tool, it only makes sense it would end up with Deadpool. The Gem appeared in a back-up story in Deadpool Vol. 3 #27 in the story "Continuity Spontaneity" by Mark Waid and John McCrea. The short story features Deadpool managing to steal the "seventh" Infinity Gem from a sleeping Thanos. This Continuity Gem gives him the power to directly communicate with Marvel Comics writers and editors, as well allowing him to move around the actual comic book and change it at will (in a manner similar to how the Unbelievable Gwen-Poole would do years later.)
Deadpool uses the Gem to quickly whisk himself and a dancer named Genosha to Las Vegas. He even teleports Siryn with them, so she can officiate their wedding. But when his new wife is shot in front of him, he realizes that the Continuity Gem was making him relive the plot of an episode of Knight Rider. Deadpool turned the Stone over to the comic creators, who appeared in the final panel so they could carefully put the Continuity Gem inside a protective case. Like with the other gems brought up here, this was the only appearance of the Continuity Gem.