Several growth parameters that affect the core formation in Bi12SiO20 and Bi12GeO20 crystals are discussed. It has been found that rotation rate is the parameter that most strongly modifies the appearance of the core. The analysis has been carried out by means of EDAX, EPR, TXRF and interferometry, and shows that the core is a more strained area compared to the core-free region. At the same time there is a higher Ge (Si) deficiency and a higher Fe concentration, compared with the core-free area.