The Heart of Darkness is a small, black diamond first mined on the desolate world of Apokolips.
Millennia ago, it was cut and shaped and used as a tool to crush those who opposed Apokolips' ruler Darkseid. The diamond was later mystically tethered to the demonic agent of vengeance known as Eclipso.
At some point, the black diamond was brought to Earth where it eventually came into the possession of scientist Bruce Gordon. Eclipso's essence, trapped within the diamond, surged outward and used Gordon as a host for his corruptive influence. Gordon fought for years to free himself from Eclipso's control, but invariably, the Heart of Darkness always returned to his person.
At one point, the Heart of Darkness appeared to an insane Jean Loring and convinced her to become the new human host for Eclipso's power. That led to the event known as the Day of Vengeance, involving the Spectre.
As a willing recipient of Eclipso's power, Loring wore the Heart of Darkness as a broach upon her cowl. She lost it during Countdown, and Bruce Gordon came back into possession of it during Countdown to Mystery.
Links and References