This is a fragment of a large nodule with an estimated diameter of approx. 500-600mm. As far as we can tell the central part is composed of green Quartz. There may have been a cavity with free-standing crystals, but we will never know that.
It is interesting to note that the area that this was discovered in there was a lot of the green laminated compacted ash evident as well.
Green Quartz like this is called Prasiolite.