Los Angeles County sheriff’s deputy Charlie Hood is attached to the ATF, working undercover on the Iron River that flows across the U.S.-Mexican border. The diamond fillings he wears in his left canine glimmer, distracting the men who sell the illegal firearms that enable the unspeakable violence on both sides of the map. Spotting the sparkle when “Charlie Diamonds” opens his mouth is often their first step toward life behind bars.