This is Iolite, a rare gem formed on earth in the prime kindergarten, but she emerged just a few years too late, even after Amethyst, but she needed extra time to form, as homeworld had never made an Iolite before, causing her to be abandoned in the kindergarten. After she emerged, she was found by Steven, Amethyst, and Peridot a few thousand years later while they were visiting. Steven brought her back to the gems, who at first saw her as a threat, but Steven said that you had been in the kindergarten for thousands of years, and had never found a way out, as everybody had left without her. They let her stay with them, and fight alongside them as a Crystal Gem. Later on, she discovered that her weapon is a dagger, with a dark blue handle and a wave engraving on the lower part of the blade.
