A fantastic specimen of “growth-inhibited” or “interrupted-growth” quartz crystals. As the quartz crystals grew, platy calcite crystals periodically interfered and interrupted the growth of perfect quartz crystals. As the interference was only partial, the quartz crystals were able to resume their growth after the interruption, to be interrupted again, repeating the process. Looking down into the large crystal, one can see that the calcite interruptions (at least at the upper, more visible regions) were not continuous through the crystal – as the quartz resumed, interruption lines remained on the prism faces to mark the interruption, but centre of the crystal is continuous without interruption.
Lower on the piece, platy calcite crystals are present, and the smaller quartz crystal ceased to grow at one of these interruption points.
At the termination of the large crystal, one half of the main “termination” face was defined by a calcite interruption, while the rest is a more lustrous natural parallel-growth termination. And the small extension of the large crystal that ends in a perfect uninterrupted termination makes the piece.
From the John S. White quartz collection, this is a great specimen – it’s hard to imagine a better and more aesthetic specimen to show this growth-interruption phenomenon and its effects.