Steven Universetale is a mixture of the gems, and Undertale. Basically it puts gems on monsters from Undertale, and adds some OCs, and canon gems from Steven Universe.
In 2017, a homeworld expedition to Earth, lead by Aquamarine (My OC) has failed and his ship has crashed near Mount Ebott. He survived the crash, and fell underground. Most of the monster underground were gems too, for the exception of Mettaton. He met Toriel, and walked through the ruins sparing monsters, and making new friends. He ventured through the underground making friend after friend, until he met ASGORE. He fought him, but soon spared him and let all the monsters free of the underground.
Sans has a red gem, he is supposed to be a Garnet, but he is not a fusion like the other Garnet is.
Papyrus is a Zircon, even though his appearance is different, he is still the spaghetti loving, friendly skeleton we all know and love!
Undyne is a pearl, like most pearls, she has a spear. But she is still the "NGAAHHH" screaming fish we know to this day!
Alphys is a Jade, she is a homeworld scientist.
Asgore is a diamond, he is a orange diamond.
Toriel is a Orange Quartz. Even though she is a defect from the diamond authority, she is still the goat mom like, butterscotch cinnamon pie maker we all know and lover!
They're just a human.