Dumortierite - Al7BO3(SiO4)3O3 - forms where silicon, aluminum and boron meet in appropriate proportions under right conditions. It's a rare mineral that usually grows as cylindric inclusions within a quartz matrix. Nevertheless, occupancy of the lattice sites is less than 100% and isomorphic substitution of aluminum by iron and titan causes blue (only Fe) to violet to violet-red (Fe, Ti) colors through charge transfer between Fe(II)-Fe(III) and Fe(III)-Ti(IV) respectively in face-sharing octahedral aluminate chains. Isolated borate triangles complete the radially symmetrical structure that spreads from a single aluminate core. The projection along the c-axis is created on the basis of crystal structure data from Moore and Araki (Neues Jahrbuch für Mineralogie Abhandlungen 132 (1978) 231): green = B, red = O, grey = Al, blue = Si. The mineral was first described in 1881 by Eugène Dumortier.