Why should you care how a stone bends light? A large refractive index, together with light bouncing around inside the stone, helps explain why diamonds sparkle more than other materials 2. Refractive index, often written as n, compares the speed of light in empty space with its speed inside a material 3. Gemmologists usually shorten it to RI. When light enters a gemstone at a slant, it slows down and bends, and the slower it travels, the more it bends 5.
Why does this matter for sparkle? Light inside a stone can only escape if it meets the surface at a gentle enough angle. Past a point called the critical angle, it bounces back inside instead. From diamond to air, that angle is only 24.4°, so light that enters has trouble getting out 2. Cutters shape the pavilion (the lower, pointed part of the stone) so light reflects inside and returns through the top. You see this returning white light as brilliance 1.
Why do some stones flash rainbow colours? In diamond, RI is different for each colour of light, so white light splits into rays that take slightly different paths. You see these colours as fire 1. Gemmologists measure RI with a tool called a refractometer, one of the standard tests for identifying coloured stones 4. RI is not the whole story, though. Cutting is what turns a material’s properties into the way the stone looks, as explained in Diamond Cut Explained 6.
Example: Picture a diamond ring under a shop light. Light enters through the top, bounces off the angled facets (the flat, polished faces) underneath and comes back to your eye as bright flashes. When some light escapes through the stone instead of coming back to you, jewellers call it leakage or windowing 6.