This strange rock is composed of a matrix of quartz and tourmaline - usually called 'schorl' by geologists, with crystals of what appear to have been feldspars in it.  There is considerable debate over whether this type of schorl was originally a fluid or whether it started off as a more normal granitic rock and was then transformed into the schorl by boron-rich fluids permeating through the rock. When borosilicate fluid cools slowly it can form a coarser grained mass composed of quartz and tourmaline - as at Roche Rock.  There is free access to Roche Rock, which is just to the east of the village of Roche on the road to Bugle (NGR SW 991/596).  It is surmounted by a picturesque medieval chapel dedicated to St Michael, which is reputed to have been inhabited by a hermit, possibly someone with leprosy.

The 'feldspars' look as if they are aligned by flow, which may be an indication that the borosilicate mass started off as a fluid.  Closer inspection of the 'feldspars' shows that, although they may have started off as feldspars, they are now composed of a fine grained mass of mica and quartz.  This is an alteration process known as greisening and crystals which have been changed in this way are known as 'pseudomorphs'.  The outlines of the crystals suggest that the original feldspars were originally twinned.  How perfectly formed feldspar crystals came to be incorporated into a schorl mass remains a mystery.  A later quartz vein cuts through the specimen.