The gemmological properties of black star sapphires from Australia and Thailand most closely resemble those of the Gold Sheen sapphire, including the high Fe content, lack of UV fluorescence, healed fractures and polysynthetic twinning
. As there are no volcanic source rocks in the reported mining area for the Gold Sheen sapphires, their geological origin remains enigmatic.
The main primary minerals include plagioclase, clinopyroxene, orthopyroxene, amphibole and minor quartzdescribed as under: Plagioclase: Plagioclase crystals are euhedral to subhedral, lathlike and columnar in shape, and exhibit polysynthetic twinning (according to the albite and occasionally to the combined albite and Carlsbad laws) and oscillatory zoning.
Orthopyroxene phenocrysts occasionally show green to pale brown pleochroism and exhibit polysynthetic twinning and locally developed reaction rims of hornblende.