The Jewel of Orthon is a gem shaped similar to a scrying crystal that Rennek borrowed from Alexi. It can be used for controlling the actions and words of people. It is unclear if the person being controlled remembers the time during which they are controlled.
Rennek borrowed it from Rennek for some unknown use that is involved in his plan to tear the Halliwell family apart from the inside. When he said he needed it for a little longer, an image of Knox was shown in the mirror, implying that either in an earlier part of his plan or the next part Knox was somehow involved. Since the jewel was later used to control someone, most likely Rennek controlled Knox during the time he was conversing with the Charmed Ones and possibly earlier when he was in contact with Tyler and when he left his card at the Mitchell's.
Rennek and Alexi used the jewel on Bailey to get her to suggest that Paige go to Salem to try and recruit Sarah for magic school. It is unclear if they were actually in magic school at the time of is they were in Alexi's Lair, but since Alexi's mirror could be seen behind them, they were probably in Alexi's Lair.