What is refraction in gemstones?
By Jared James · Last updated 21 May 2026
Definition
Refraction is the bending of light as it passes from one transparent medium into another, for instance from air into a gemstone, and it is central to how gemstones sparkle, disperse colour and appear brilliant. Each gem material bends light at a characteristic angle, described by its refractive index. Diamond has an exceptionally high refractive index of around 2.42, which is why it bends and reflects light so dramatically, while glass has a much lower index of around 1.52. Gemologists use a refractometer to measure this index as one tool for identifying unknown stones.
Frequently asked questions
- Why does refraction matter in diamonds?
- A diamond's high refractive index means light bends sharply as it enters the stone, bounces around the pavilion facets and exits through the table as brilliance. If the proportions are cut correctly to take advantage of this index, the result is the intense sparkle diamonds are known for.
- What is a refractive index?
- A refractive index is a number describing how much a material bends light relative to its speed in a vacuum. A higher number means the material bends light more strongly. Diamond's index is 2.42, moissanite is 2.65 to 2.69, sapphire is 1.76 to 1.78 and glass is around 1.52.
- Can refraction help identify a gemstone?
- Yes, measuring the refractive index is one of the first things a gemologist does with an unknown stone. Most gem species have a distinctive index or range, and combined with other properties like density and optical character, it helps identify the stone.
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