Between 290 and 270 million years ago a great mass of molten granite magma rose up into the core of a newly created mountain range, situated where Cornwall and Devon are today, and slowly cooled and crystallised.  The mountain range was eroded away by the weather a long time ago and the underlying granite is now exposed at the surface on Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. At depth, granite underlies much of the peninsula of Cornwall and West Devon.  The commonest type of granite is biotite granite, so named because it contains the dark coloured mica called biotite.  This specimen also contains large white crystals of feldspar and this variety of granite is often known as 'big feldspar granite'. 

All granites consist of three basic minerals - quartz, feldspar and mica.  The commonest type of granite contains an iron bearing mica called biotite, which is dark brown or almost black.  Two types of feldspar are present in this specimen - potash feldspar (usually orthoclase or microcline, often microperthitic) and a soda feldspar (plagioclase - albite).  The potash feldspar occurs both in the groundmass and also as prominent big white crystals up to 10 cm long which are called megacrysts.  Kaolinisation is the process whereby the feldspars in granite are converted to china clay; biotite granite is not an ideal parent for china clay because the iron contained in the biotite mica is released during kaolinisation and tends to stain the china clay red or brown.  However, some workable deposits of china clay, such as those on Bodmin Moor, were derived from biotite granite.  A thin bluish/black tourmaline vein traverses the specimen, note that the granite on either side of the vein has been slightly altered  This is a process known as greisening, where the feldspar is altered by the fluids circulating along the vein to a mixture of quartz and mica, this process is also seen in several other boulders.  This specimen comes from Goonbarrow china clay pit and has one of the highest amounts of radioactive minerals of any Cornish granite.  All the Cornish granites contain abnormally high contents of uranium and thorium, which not only release radioactivity, but also heat.  It is this heat which helped to considerably prolong the cooling of the granite, causing water within the granite and the surrounding rocks to circulate by convection, leading to the formation of tin and copper mineral deposits and causing kaolinisation.  One unwanted by-product of radioactivity is the release of radon gas, which can seep into people's homes and cause a cancer risk.  Granites similar to this specimen were extensively worked in the Luxulyan valley in the 19th century and many famous structures, including the old London Bridge and Plymouth Breakwater, were constructed from this type of granite.