The only tanzanite deposit on our planet barely covers eight square miles, and “D Block” refers to one of the four blocks into which it was divided by the Tanzanian government. Legend has it that D Block is responsible for producing the finest tanzanite. However, the scientific truth is that tanzanite’s quality or value can in no way be guaranteed by its exact location within the deposit.
Nevertheless, many online sellers try to capitalize on this legend and use the phrase “D Block” to promote their tanzanite as superior. You should be aware there is no scientific way to tell which block any tanzanite gemstone came from (as it is all one deposit), and, even more importantly, that the block is in no way an indicator of high quality.