The Jewel of Jahari is a shell/cup game run by Aadan in Aleph's Black Market, during the mission The Past Never Dies (Mauritania). It's rigged — but the Q-Lens beats it every time if you use it at the right moment.

The trick is to tag the jewel with the Q-Lens while it's still being shown to you — not after the shuffle.
Talk to Aadan in the Black Market and pay the $3,500 buy-in to start a round.
As Aadan first shows you the jewel, hold the Q-Lens — L1 (PS5), LB (Xbox) or Alt (PC). The jewel highlights orange so you can track it.
Aadan's sleight of hand is rigged — eye-tracking alone always loses. The orange Q-Lens tag is the only reliable marker.
Once you've tagged it during the reveal, the jewel always ends up under the middle cup. Choose it to win.
Take the Jewel of Jahari to Nirmala at the pawn shop — she pays $30,000, close to a third of your auction target.

If you only raise the Q-Lens after the cups stop moving, the scan won't register the jewel. You must scan during the opening reveal.
Trying to follow the cups by eye can't beat a rigged shuffle — without the Q-Lens tag you'll lose your buy-in every time.