Unity is a race of blue crystalline lifeforms found on P3X-562. When the Goa'uld Kur visited their planet long ago, Unity tried to greet him. Kur touched one of the crystals and was killed. In retaliation, the Jaffa gathered Unity in one place and attempted to destroy them with staff weapons. Only a few crystals survived, almost none of them whole.
In 1997, SG-1 visited P3X-562. While there, Jack O'Neill touched an intact crystal which rendered him unconscious. In fear of another retaliation, the crystal took O'Neill's form in hopes of fixing his pain. However the greatest pain it found in O'Neill wasn't physical, but emotional; the death of his son Charlie. Not understanding what death meant, the crystal Jack went to Earth looking for Charlie to bring him back to O'Neill. Samantha Carter brought some of the crystals back to Earth for study, however one of the crystals made contact with them, telling them Unity's history. Through further study, Carter found that the crystals couldn't survive in the Earth's stronger electromagnetic field and released radiation as a result. The crystals along with crystal Jack, then in the form of Charlie, were returned to the planet. (RPG: "Fantastic Frontiers: Stargate Season One") (SG1: "Cold Lazarus")