I have the same rock. It made a mild signal on my SDC one day so I brought the big sucker home. There were other rocks the same, but only this one was giving a signal. It has quartz through it and has some shiny silver specs through it (pyrites maybe). I kept it anyways.
I will upload a better pic later but here is one I have on my phone.
I agree with the sedimentary comment. It does seem to be layered for sure. The quartz is very white, like it has totally permeated through the shale or clay or whatever it was. I am no geologist. Some parts of the rock had darker concentrations like the pic I uploaded, some other parts seem to be more like the original pic uploaded, but all had some silver substance within its layers.
The second photo does look like quartz, but the first photo almost looks like there is cleavage which would make me think calcite. Is the white part soft, can it be scratched with a penknife or does the knife leave a mark on the rock?
I would say that it is quartz and the material around it is sheared sediment that has largely now been heated and stressed to form chlorite. This is a typical vein structural feature known as a boudin. They are common in Victorian slate hosted gold deposits.