After the granite had crystallised and lost its initial heat, stresses in the granite and its surroundings caused cracks to form.  Water from the still cooling granite and the rocks into which it was intruded now began to circulate through these cracks.  This hot water contained iron, silica, boron and other chemicals in solution.  As the whole mass cooled, these chemicals combined to form the minerals tourmaline and quartz, which crystallised in the cracks in the granite to form the bluish-black material in this boulder.  Often the crack also acted as a fault and movement caused the material in the forming vein to be broken up and crushed, which can also be seen in the boulder.  On one side a portion of the granite wall alongside the vein can be seen.  In places these quartz tourmaline veins carried significant amounts of the tin mineral cassiterite.  This was worked in underground mines and open pits; sometimes these 'old mens workings' are seen in china clay pits as they expand and deepen.

This boulder is a very good example of the type of quartz-tourmaline vein found in china clay pits.  Clearly it is composite, with much evidence of movement.  Small slivers of the granite can be seen to have been sheared off and incorporated into the vein.  Both these slivers and the wall rock on one side of the boulder have been affected by greisening - a process whereby the original feldspar has been altered to a mass of quartz and mica by solutions rich in silica, no doubt emanating from the vein.  The latest phase of mineral deposition laid down the white quartz with practically no tourmaline.  In places the quartz crystals grew into cavities (vughs) and the pseudo-hexagonal form of the quartz crystals can be seen.  Studies of small inclusions of fluid trapped inside the forming crystals can tell us about the fluids responsible for their formation.  They were hot (in the range 350-450^oC) and quite salty, sometimes minute cubic crystals of salt can be seen floating in these fluid inclusions under the microscope.  Tin workings in china clay pits include:  Beam nine in Goonbarrow china clay pit (underground mine),  Rocks mine in Rocks china clay pit (underground mine) and Carclaze open pit mine, worked since Elizabethan times. This last pit forms part of the present day Carclaze china clay pit and can readily be seen from the newly constructed trail leading from Wheal Martyn to the Eden Project (NGR SX 025/550).  At present day values this small pit at Carclaze (about 13 acres - 5.25ha) yielded £100million worth of tin in present day values.