What is the typical dispersion value (fire) for diamond?

• A. 0.010
• B. 0.044
• C. 0.100
• D. 0.060

Answer: (B)

Dispersion, or fire, is how much a gem splits white light into colors because the index of refraction changes with wavelength. In diamond, this change is significant enough to produce noticeable spectral colors, yet not so extreme as to overwhelm the stone’s light return. The standard way gemologists express this property is the difference between the refractive indices for red and violet light, often written as nF minus nC. For diamond, that difference is about 0.044. This value is the benchmark you’ll see cited for diamond’s dispersion. The other numbers are outside the typical range you’d expect for diamond’s fire, either being too low or too high. So the typical dispersion value for diamond is 0.044.