Gahnite is a scarce zinc aluminum oxide mineral (ZnAl2O4). It principally occurs in some igneous & metamorphic rocks. It has a nonmetallic luster, an extremely dark green to x. dark bluish-green color, no cleavage, and is very hard (H = 7.5 to 8). The only gahnites I've ever seen are in the two pegmatitic granite samples shown below from near Broken Hill, Australia.
Gahnite (dark-colored) (above & below; above: field of view 2.0 cm across; below: 1.6 cm across) in coarsely-crystalline granite.
Age: early Mesoproterozoic.
Locality: Southern Cross, ~11-12 km north of city of Broken Hill, Broken Hill Block, far-western New South Wales, southeastern Australia.
Collected & generously donated by Molly Tannian.