Opaque quartz has air injected into the quartz process, giving it an opaque color and insulating properties. Opaque quartz is traditionally used in scientific glass to stop the transfer of heat radiating from the fiber optic effects of the the clear quartz (quartz can transfer the heat from the heated portion of the tube to the end that's not being heated). Opaque quartz stops heat by stopping light going through it and the air bubbles insulates and pushes heat away, thus making it colder than clear. This is why you see people using giant chunks of opaque quartz- it barely holds heat.
So the idea here is the concentrate stays in the cold part of the glass (opaque) to allow the preservation of the terpenes. As you pull on the piece, the oil will go to the clear hot quartz outside the center of opaque to give you a fresh non-degraded flavor punch.