What is the Master-Eye Effect?

• A. A measurement of fluorescence.
• B. The optical illusion that causes the ungraded diamond to appear either lighter or darker than the masterstone, depending on which side it's on.
• C. A method for calibrating color using a master reference.
• D. A type of cut used in grading.

Answer: (B)

The Master-Eye Effect is a visual illusion that occurs when you compare an ungraded diamond to a fixed master stone. Because the eye and brain compare two adjacent stones under the same light, the ungraded diamond can look lighter or darker than the master depending on which side is oriented toward the viewer. This is a perceptual bias, not a real change in the stone’s color or clarity, driven by contrast and lighting conditions. Graders work around it by rotating stones, using consistent lighting, and comparing under the same viewing setup to minimize bias. So the phenomenon describes how the appearance of the ungraded stone shifts relative to the masterstone, not measurements of fluorescence, calibration methods, or a cut type.