This small boulder shows purple fluorite (calcium fluoride) forming a thin layer on a joint in a granite superficially resembling boulder no. 4.  However, the presence of tourmaline in the granite would make this granite a much less attractive material than boulder no 4 for ceramic use.  The high fluorine content of some of the St Austell granites means that two fluorine-bearing minerals are found: topaz and fluorite.  Topaz is commoner than fluorite because the St Austell granite is low in calcium and fluorite (CaF2) requires calcium.  This fluorite was probably formed at a late stage in the cooling history of the granite.