How Fast Is Light in Diamond?

Light slows to 124,000 km/s in diamond — about 41% of vacuum speed. This dramatic slowdown is what gives diamond its sparkle: light bends sharply as it enters and refracts internally before escaping, creating the characteristic flash.

Light slows to 41% of vacuum speed in diamond — which is exactly why diamonds sparkle the way they do.

• The slowdown creates diamond's sparkle
• Diamond has the highest refractive index of any common gem