Opaque quartz has air injected into the quartz process. This gives it an opaque color and unique insulating properties. Opaque quartz is traditionally used to stop the transfer of heat that radiates from the fiber optic effects of clear quartz.
In simple terms, the opaque quartz stays cooler than any surrounding clear quartz. This is why you see people using giant chunks of opaque quartz- it barely holds heat.
We use opaque quartz on our bangers because it gives a "cool spot" for oils to rest in the banger. When air flows through a carb carb, it forces the oils to the hotter side walls of the banger.