The answer to whether diamonds make rainbows is yes, but the phenomenon is technically known as “fire” or “dispersion.” This effect is the dazzling display of colored light that flashes from a diamond as it moves. Diamond fire occurs because the gemstone acts like a prism, separating white light into its colors. This quality contributes to the diamond’s reputation for intense sparkle and visual appeal.
The Science Behind Diamond Fire
The colored flashes, or fire, result from two physical properties: refraction and dispersion. Refraction occurs when light enters the diamond and slows down, causing it to bend inward. Diamonds possess a high refractive index (approximately 2.42), which measures how much the light is bent.
This high refractive index helps trap the light inside the stone. White light is composed of all colors, each possessing a slightly different wavelength. As light travels through the diamond, these wavelengths separate because they are refracted at slightly different angles.
This separation of white light into its spectrum of colors is known as dispersion. The diamond’s dispersion rate is approximately 0.044. Once the separated light reflects off the internal surfaces and exits the diamond, it appears to the viewer as colorful flashes.
The Role of Cut and Clarity in Maximizing the Effect
The ability to create fire is an inherent property of the diamond material, but the stone’s cut is the most important factor in realizing that potential. The way a diamond is shaped and faceted determines how effectively light is captured, dispersed, and returned to the eye. A well-executed cut balances brilliance (white light return) and fire (colored light return).
The round brilliant cut is engineered with 57 or 58 facets to maximize both light return and dispersion. If a diamond is cut too deep, light leaks out of the bottom, known as a “fisheye” effect, which reduces brilliance and fire. Conversely, a cut that is too shallow allows light to escape through the sides, making the stone appear dull.
The angles and proportions of the facets, particularly the pavilion and crown, must be in harmony to ensure light reflects internally before being dispersed through the top. Clarity also plays a role, as internal flaws (inclusions) can scatter or block the path of light. Fewer inclusions allow for a more uninterrupted flow of light, enhancing the performance.
Comparing Diamond Fire to Other Gemstones
Diamond fire is contextualized by comparing its dispersion rate to other gemstones and simulants. Diamond’s dispersion value of 0.044 provides a distinct optical quality. Some diamond alternatives exhibit a higher dispersion rate, resulting in more noticeable colored flashes.
Cubic zirconia (CZ), a common diamond simulant, has a dispersion rate around 0.060. Moissanite, another popular alternative, demonstrates greater fire, with a rate of approximately 0.104. Moissanite separates light more effectively than a diamond, producing intense rainbow flashes described as a “disco ball” effect.
Despite the higher dispersion of simulants, diamond is prized for its combination of high fire, high brilliance, and hardness. This balance of optical properties and durability results in a visual effect distinct from the exaggerated fire seen in moissanite. The appeal of a diamond lies in the harmonious balance of its white light return and its subtle, intense colored flashes.