corundum, synthetic corundum.7, gems

Twinned individuals of Ramaura synthetic rubies were examined and identified as penetration twins, with the two intergrown crystal components symmetrically related either by a 180° rotation about the c-axis, by reflection across one of the first-order hexagonal prism planes {101̅0}, or across the basal pinacoid (0001); the twins are in contact along four twin boundaries aligned with these twin planes, which together delineate a twinning form never before observed in either natural or synthetic corundum. Each twin crystal exhibits a single planar surface that marks the interface between the ruby and the floor of the crucible used during the growth process; these surfaces are divided into multiple sectors by the intersecting twin planes, which converge at a central point. On the basal planes of these crystals, triangular growth structures are seen to relate through the twin symmetry elements—specifically, a two-fold axis parallel to [0001] and mirror planes parallel to {101̅0}.