The Magatama

The Magatama is a jewel from the Ace Attorney canon. However, it is no ordinary gem-It has magic within it that allows one to see Psyche-Locks, the locks that guard one's secrets that they are currently hiding from others. The deeper the secret, the more Psyche-Locks exist. However, they only appear when someone is lying-It can be tricked through things that are technically true, as the case of Matt Engarde shows.

Once the locks are broken, the target tends to be more willing to divulge in more details. This is probably because the core part of the secret has already been found and proven at this point.

There are also black Psyche-locks, which represent a secret kept subconsciously-That is, the one with them aren't even aware of it.

The Magatama is currently with the Nightmares.

Trivia

 * Considering that the Nightmares got the Magatama in May of 2018, which would be not long after the ending of Justice For All, while it was still used by Phoenix later on according to Ace Attorney canon in Trials and Tributions, it's possible that canon has been broken. Though this could also simply be that the years of the Ace Attorney games do not align with the years of World Prime, just like Star Wars. Or maybe Maya had to give Phoenix another Magatama, and that's why the mechanics differ slightly between games.