Star sapphire asterism orientation: cutting cabochon domes along the optic c-axis?

Asterism in Ceylon star corundum (sapphire and ruby) is caused by dense networks of microscopic rutile (TiO2) needle inclusions oriented along the three crystallographic a-axes in the basal pinacoid plane, intersecting at 60-degree angles. To produce a perfectly centered six-ray star, the lapidary must orient the rough stone so that the optic c-axis is strictly perpendicular to the flat base of the cabochon. When shaped into a symmetrical oval or round cabochon with an ideal dome height-to-width ratio (roughly 1:2), the curved convex surface acts as a lens. Reflected light off the three sets of rutile silk focuses into three sharp intersecting lines that converge directly at the dome apex.