Cocoa butter, the fat component of chocolate, is polymorphic; meaning it can solidify into different crystalline forms at different temperatures. It is also monotropic, which means that over time, all crystals will transform to the higher melting point crystals, or b crystals.
Chocolate therefore needs to be tempered to gain an adequate proportion of stable b crystals in the liquid chocolate before it is used in coating or moulding.
If the chocolate is not tempered, cooled and stored correctly, b’ (beta prime) crystals may form, later transforming to large b crystals, appearing on the surface of the chocolate as a white hazy substance, or ‘bloom’.
Many cooks, unfamiliar with this condition believe the chocolate has become moldy, often discarding perfectly good chocolate for no reason. Simple tempering will restore the chocolate to a proper appearance.