A harmonic diamond is a type of extremely valuable gem stone which generates an audible sound. Some harmonic diamond also generate low-level energy waves which stimulate areas of the brain associated with primitive emotions, including anger, envy, and greed.
In, or shortly after, 2296, a group of criminals came into possession of a collection of harmonic diamonds crash landed on the planet Rajatha Prime. Before they could do anything about there predicament they killed each other.
In 2368 an archaeological expedition discovered the criminal’s crashed ship and the diamonds. By the next day all but two of the team were dead, killed by other members of the expedition affected by the diamonds. Per the team leader's final request the diamonds were used to fund archaeological digs across the quadrant. (TNG comic: "Captain's Pleasure")
- It is implied this incident was part of a larger conspiracy, so the diamonds were likely planted for the Maquis to find them, and soon after destroy themselves.