The Trial is a particular court session on Homeworld, taking place shortly after Blue Diamond was taken away by Grootslang. The matters concerned that and the involvment of Pink Diamond's shattering against the Crystal Gems.
The trial itself began on the same day that Blue Diamond's supposed "shattering" took place. White Diamond went on to set up the trial, placing Zircon for the Crystal Gems as defense, and a second Zircon for prosecution. The session began with the prosecution going over the previous crimes Rose Quartz had on the Gem War. She proved Steven was Rose by triggering his shield, and later was called for false conclusions by Fulgurite. The Defense counteracted this with the lack of knowledge the guilty party had of said case, and called in Blue Pearl as a chief witness. The first session was cut short when White Diamond broke down during the session.
After a short recess, a extra witness, Flint arrived in aid of the guilty party. The defense side called on major evidence in that, time wise, it would be impossible for Steven to go to Homeworld and take her away, and even brought in the Blue Diamond gemstone and palanquin as a presentation, proving that the Gemstone wasn't even a gemstone at all, and simply made up of reptile scales.
The final verdict was banishment for the Crystal Gems, and if they were to return, they'll be placed for harvesting.