What makes one diamond beautiful and another one dull is the way it reflects and refracts light, better known as the diamond's Light Performance.  Gemologists use imaging spectrophotometer technology to measure a diamond's Brilliance (white light), Fire (color light) and Sparkle (scintillation).

Then they rate each of these three measures compared to other diamonds of the same shape. The scale for each shape was set by measuring thousands of diamonds to determine the highest and lowest amount of Brilliance, Fire and Sparkle possible for each shape.

Premium diamonds have High-Very High ratings, Average diamonds have Medium-High ratings, and Discount diamonds have Low-Medium ratings.

Diamond Light Performance Scale
The diamonds to the right are all within the same 4C's category.
the chart below represents the measurement of the
 bottom diamond (HIGH to VERY-HIGH)

.
BRILLIANCE (White Light) ι     LOW     ι     MEDIUM     ι      HIGH     ι      VERY HIGH     ι

FIRE (Colored Light)           ι     LOW     ι     MEDIUM     ι      HIGH     ι      VERY HIGH     ι

SPARKLE (Scintillation)     ι     LOW     ι     MEDIUM     ι      HIGH     ι      VERY HIGH     ι

Based on a Spectrophotometer measurement of an actual diamond, but for educational purposes only.