"It is not a conflict diamond," Wentz, 28, told PEOPLE Wednesday night at the NME Awards USA at Goldenvoice's El Rey Theater in L.A.
Conflict diamonds typically refer to diamonds that are sold to fund unlawful or illegal operations of rogue military, rebel and terrorist groups.
"It's something I care about," Wentz added. "I can't actually go [to Africa to pick it out] myself, but I trust [the jeweler]. He's a reputable person."
At the NME Awards, the Fall Out Boy bassist donned a T-shirt emblazoned with the message "Coltan is a Conflict Diamond," referring to the metallic ore largely found in the African nation of Congo.
But with his marriage to Simpson, 23, slated for May in California, say sources, is Wentz too busy trumpeting social causes to be involved with wedding plans?
"I'm involved, but in a peripheral kind of way," he said, laughing. "I know who's in charge!"
Reporting by MARISA LAUDADIO