Interesting speci there. I'm hard pressed to guess if you've a phantom crystal or if indeed a quartz crystal formed over a bit of aqua. I believe GFreeman had a crystal up a bit ago that looked to be similar~ quartz over beryl if I remember right.
When conditions are right quartz will grow over many materials. I've had/have species with copper, gold, manganese crystals inside the main body as well as a few phantom crystals. I can not rightly say how you'd test it *scratches head* but its a keeper.