What is the approximate refractive index range for diamond?

• A. About 2.417–2.419.
• B. About 1.50–1.60.
• C. About 2.00–2.10.
• D. About 1.30–1.40.

Answer: (A)

Light bending and slowing is quantified by the refractive index, a measure of how much light slows down as it passes through a material. Diamond has a very high n in the visible range because its crystal structure and strong covalent bonds cause light to slow down more than in most other substances. The visible-range refractive index for diamond is about 2.417–2.419, which is why it exhibits remarkable brilliance and sharp internal reflections when well cut. This high value is much larger than materials like common glass, which sits around 1.50–1.60, or other gems that have indices in the 1.7–1.8 range; those materials don’t produce the same level of sparkle. The exact figure can shift slightly with wavelength, but in the visible spectrum diamond’s index stays near 2.417–2.419, making it the standout among gem materials for refractive power.