After the granite magma had moved into place and started to cool, stresses in the earth's crust caused deep cracks to form, reaching down many kilometres to the molten granite at depth, which rushed up the cracks to form narrow vein-like bodies of 'elvan'.  As these were relatively small (usually only a few metres wide), they cooled rapidly, giving the quartz, feldspar and mica crystals very little time to grow.  This results in a fine grained rock called elvan.  Some of Cornwall's finest building stones are elvans - look at the wonderful medieval carvings on the tower of St Austell Parish Church.

Elvans typically occur as igneous dykes and may extend a considerable distance from the granite mass to which they relate.  They were formed several millions of years after the molten granite moved into place (usually in the period 265-260 million years ago).  Granites cool very slowly and the top would have cooled and crystallised at the same time as the granite at depth was still molten.  Cracks formed by stresses in the earth's crust tapped into this deep reservoir of molten magma allowing some to rush up towards the surface, perhaps also causing volcanic activity on the land surface above.  Some geologists believe that the elvans were fluidised into place as a suspension of fine magma blebs in hot gas. Elvans have the same chemical composition as granites but, because they cooled relatively quickly, they are much finer grained.  In some cases they appear to have cooled so rapidly that there was no time for crystals to form and they cooled as a glass, which has slowly devitrified over geological time.  Very often flow banding can be seen, particularly near the margins.  Two generations of quartz veins cut through this specimen, more or less at right angles, indicating that the stress conditions changed in the time between the two sets of veins being formed.  The quartz veins are stained red by iron oxide - probably in the form of the mineral haemetite.  Elvans can make good building stone, providing they are not softened by the kaolinising process.  This boulder is from Wheal Remfry china clay pit, and has been slightly altered, so would not be suitable as a building stone.  Where the elvans are found some distance from the granite, they tend not to be kaolinised and can make an excellent 'freestone'.  Freestone is a rock that can readily be carved into an intricate shape.  Examples are Pentewan Stone used for St Austell Parish Church and Newnham Stone used for many of Georgian buildings in Lemon Street, Truro.