Characteristic of Al-rich, Si-poor geological environments; in
syenite and
monzonite, and some quartz-free
pegmatites; primary or a reaction product in
eclogitic xenoliths in
kimberlites. In regional or contact
metamorphic aluminous rocks, and some advanced
argillic and potassic hydrothermal alteration assemblages (
Anthony et al., 2001—2005). Famous specimens:
Black Star of Queensland, 733 carats from Australia, now owned by an unknown private party;
Logan sapphire, 422.99 carats from Sri Lanka, in National Museum of Natural History, Smithsonian Institution, Washington.
