Gahnospinel was originally reported from the gem gravels of Ratnapura in Sri Lanka. It is now known to also occur in the alluvial deposits from Tunduru, Tanzania albeit in very limited quantities. It looks like spinel, is singly refractive like spinel and falls within an identical color range. From appearance, its impossible to differentiate Gahnospinel from spinel but its specific gravity is somewhat higher.
From our investigations of mine run Tunduru spinels, the odds of finding a blue or a blue-green Gahnospinel seem to be in the order of something like one in ten thousand so these stones are quite rare. Black gahnospinel is either much more common or misidentified because it is being offered online from an undisclosed source in calibrated sizes.