Why Do Diamonds Sparkle More Than Rubies or Glass?

Several factors make up the sparkle of gemstones and other minerals. The combination of refraction and dispersion, combined with the item's form, work together to create sparkle. The amounts of refraction and dispersion differentiate one gemstone or material from another.

Background

A ruby heart

All materials interact with light. Opaque materials reflect certain colors of light back to the eye, and that is how we perceive them. Transparent materials don't just reflect light, they also bend it and break it down. The level to which this happens creates a signature perception of that material. The first thing that comes to mind when someone thinks of diamonds is the distinct flash and sparkle. The first impression of a ruby is in the depth of its color. Glass, of course, is known for its clarity, not its sparkle.

Refraction

Refraction of water

Refraction is the bending of light as it passes from one medium to another. The best example of this is the classic school experiment of putting a straw into a glass of water and seeing the distortion. All gems and minerals refract light. The measurement of this bending is known as the index of refraction. The higher the refractive index, the more the light bends. Diamond has the highest refractive index (RI) of all transparent minerals, at 2.417. A ruby's RI is 1.766, and the RI of common window glass is 1.52.

Dispersion

Dispersion

Dispersion occurs when light is broken down into its component colors. Think of a prism hanging before a window; as light passes through it, the prism creates rainbows on the glass and walls nearby. Those rainbows are the result of dispersion. All gemstones and clear materials have dispersion. The dispersion of diamond is very high, at .044, followed by ruby at .018. The dispersion of glass is so low it is considered negligible.

Cut

Over the centuries, diamond cutting has been turned into an art form. In 1919, Marcel Tolkowsky, a Belgian-born gemologist and mathematician, developed the "ideal" cut for diamonds that remains in use today. This cut calls for a specific number of facets to create maximum refraction in diamonds, to increase sparkle. Rubies are often cut with a round "belly" to maintain as much weight as possible; the result is an even lower refraction than naturally occurs in the mineral. Glass, of course, comes in all shapes and sizes.

Sparkle

Taken together, three factors create sparkle. Diamond's refraction and dispersion are both higher than for either ruby or glass. Ruby's refraction and dispersion are higher than those of glass. In addition, the cut of diamonds has been refined over centuries to create the maximum sparkle possible; add in the refraction and dispersion values, and the diamond clearly outshines rubies and glass.

References

• "Diamond Design"; Marcel Tolkowsky; 1919
• "The Book of Diamonds"; Joan Younger Dickinson; 1965
• Encyclopaedia Britanica Online
• Gale Virtual Reference Library
• San Diego State University: Dispersion