Pleochroism

The property of showing different colours when viewed along different crystal directions. Sapphire shifts between violetish blue and greenish blue; tourmaline can change dramatically. Cutters orient the rough so the best colour faces up, which is part of why orientation is a valued skill and poorly oriented rough sells at a discount. On a finished stone, rock it under light: colour that shifts oddly across the face is often pleochroism showing through the cut.