This earring appears as a small black stone set in a silver setting. The stone has veins of red running through it. Once used it cannot be used again for one week... or perhaps you could trade something to reactivate it sooner. Knowledge is power... and currency.
While wearing this earring, you can use an action to make an insight check against a target creature you can see. On a success the earring whispers something to you about the target - a dark secret, a hidden weakness, a phobia. This information can be used to give advantage against the creature on your next turn based on the knowledge provided.
On a failed save, the earring tells you one of your own dark secrets which unsettles you, giving you disadvantage on your next action.
The idea behind this item is that otherworldly beings can communicate with you through the stone. These beings can identify things in others that haunt them, or can otherwise be used against them. When they provide you with insight into your target it can provide advantage for your against them specific to the type of information provided. If it is a deep fear, you can use that to try an intimidate your opponent. A dark secret may unnerve the target, opening them up to persuasion. A hidden weakness could give advantage on the next attack.
But beware, those same beings can see into your own dark thoughts and hidden fears. What they know about you can be very unnerving.