Refractive index and fire
The higher a stone's refractive index and dispersion, the better its fire and the brighter it looks. Lab-grown diamonds are the same as mined diamonds here, with a refractive index of 2.42 and a dispersion of 0.044, both extremely high among gemstones.
Cut style
The classic round cut, also called the brilliant cut, is the style that best shows a diamond's brightness and sparkle, and it is one of the most popular shapes on the market. Other cuts with high sparkle include the oval, pear, marquise and princess.
Cut quality
The better a diamond's proportions and polish, the more fire and sparkle it has overall. At GIA and HRD the top cut grade is Excellent; at IGI the top grade is Ideal, followed by Excellent. Choose a 3EX cut where you can and the diamond will be very bright.
Clarity and light
Under the 4C standard, flaws at SI clarity and below are fairly noticeable, and in serious cases they affect how well the diamond transmits light, which naturally reduces its sparkle. A diamond of VS2 clarity or above releases more of its light and sparkles more.
