Gemological Institute of America (GIA) Graduate Diamonds Practice Exam

Pipe deposits come from explosive volcanic activity that creates vertical conduits through the crust. These pipes are typically filled with or lined by kimberlite or lamproite, ultrabasic igneous rocks that originate in deep mantle sources and can transport diamonds to shallower depths. That combination—kimberlite and lamproite forming the pipe structure—is what makes pipe deposits diamond-bearing in many cases.

Think of it this way: a pipe is not a horizontal sedimentary layer, nor is it a metamorphic rock. It’s an igneous volcanic channel, and many such channels host diamonds because the kimberlite/lamproite magma carries them from depth and concentrates them as it intrudes and erupts.