Vic, a little more info, from “Gems made by man”, Nassau, 1980n
the bits enclosed by [[double brackets]] are my additions to the
"StructuRe: the (alpha) Al2O3 corundum structure is based on a
near hexagonal close packing of oxygens, which occur in sheets,
as shown in figure 8-9 [[sorry, you’ll have to look this up
yourself if you need the diagram.]] . Between the oxygen sheets
there are somewhat distorted octahedral holes, and two out of
every three of these octahedra are occupied by aluminum in a
regular pattern. Each Al is thus surrounded by six O.
Crystal symmetry: Hexagonal, class [[bar over the 3]] 32/m;
space group R3c [[bar over the 3]]
Optical properties: Refractive index o = 1.770, c = 1.762.
(green sapphire may be up to .01 higher); birefringence 0.008,
dispersion 0.018; unizxial negative.
Hope this helps a bit more.