Diamond was a drug derived from I.G.H. developed for Diamondback's men for him to sell. Following Diamondback's fall at the hand of the Defenders, The Hood took control of his territory and operations, and took Diamond off the streets, deeming it crap.
Once ingested, Diamond produces a second-long surge of energy in its consumer (with a glow coming from eyes and mouth), followed by an unstoppable rush that lasts for hours. Even though at first glance, Diamond doesn't appear to have negative side effects, apparently mixing it with other drugs causes death.