What is Fire in a Diamond?

What is Fire in a Diamond?

By Claudia S. Copeland, PhD 

Diamonds are famous for the myriad dazzling colors that play across their surface as they catch the light.  These vivid flashes of color are called fire, and of all the precious and semi-precious gems on earth, the most famous for its fire is the diamond.  So, what is it about a diamond that causes these vivid sparks of color?

Refraction, Reflection, and Brilliance

 To understand fire, we must first understand a bit about light, and how a gemstone interacts with light.  Beams of light can do two things when they interact with a substance—they can either bounce off the surface of the material or they can go through the material.  When they bounce off the surface, we see this as reflection.  When they go through a translucent material, the beams slow down, causing them to “bend” (change their angle) in a phenomenon known as refraction. 

One common example of refraction is seen when you put your arm under clear water, such as in a swimming pool: your arm will appear bent at an angle from where it enters the water.  This is because when the light moves through the more dense water (compared with air), it slows down; the water bends the beam of light.  Clear gemstones do this, too, with different gems slowing the light down to different degrees.  The extent to which they do this is an inherent property of a mineral, and therefore can be used to identify the nature of a gemstone.  The measure of how much a mineral slows down and bends light is called its refractive index.  


What goes on in a cut gem, though, is a bit more complicated.  Depending on the angles of the facets and the refractive index of the gem, some light will enter the stone, but be reflected back into the inside of the gem until it bounces out at a different angle.  The more light that does this internal dance and emerges through the crown (top), the more brilliant the gem appears. Gemologists are trained in cutting a gem to produce just the right angles to maximize the brilliance you see emanating from the crown.  

The Phenomenon of Dispersion, or How to Make a Rainbow from a Stone

 Any type of light can emerge from a gem to be seen as brilliance (and/or scintillation, the flashing or sparkling of light). However, the way that light is refracted (the refraction index) is specific to a particular wavelength (color, to our eyes).  Different wavelengths of light are each bent to different degrees within the same substance.


 White light is a combination of all visible wavelengths—a complete spectrum of light colors. When white light enters certain substrates, such as a prism or crystal, each wavelength slows down to a different speed, causing the different colored light beams making up white light to be refracted at different angles. When they emerge from the crystal, they do so not as the white light that entered the crystal, but separated into their component wavelengths—seen by our eyes as a spectrum of colors.


This process is known as dispersion, and crystals are not the only substances that can do it—water droplets can do it, too. This is the familiar but often breathtaking phenomenon we see when sunlight shines on falling raindrops—the rainbow.  Although the flashes of fire from a diamond look different from the soft glow of a rainbow stretched across the sky, it is the same phenomenon of dispersal that produces the colors in both.  In this sense, that diamond tennis bracelet sparkling on your wrist is not just a collection of static stones—that dazzling bracelet is actually creating rainbows! 

Refraction, Dispersal, and High-Fire Gemstones 

The degree of fire (dispersion) is related to the refractive index of the stone. Diamonds, famous for their fire, have a very high refractive index, but there are some stones that exceed even diamonds in their dispersion, such as cerussite and lithium niobate.  (Because of other properties, these stones are not well-suited for jewelry, though.)  Jewelry-suitable gems with high levels of fire (albeit not as high as diamonds) include zircons and sphenes (titanite). 

Displaying Fire in a Diamond: Simplicity and Elegance

Because the brilliance and fire of diamonds is so beautiful, the ideal setting for a diamond with a lot of fire is one of simplicity.  Rather than cluttering the setting with intricate designs, simple settings such as diamond stud earrings or the Tiffany solitaire ring design, in which a single diamond is mounted on prongs, are best when it comes to displaying the breathtaking beauty of a truly fine diamond in an engagement ring.  The diamond stud can be a joyous and playful way to show off fire, since diamond studs subtly frame the face with the lively movement of rainbow scintillation.  


SuperJeweler has beautiful diamond jewelry at a range of prices to fit any budget.  From diamond stud earrings to our fiery diamond solitaire necklace, our selection of diamond jewelry is sure to include the perfect “rainbow in a stone” for you or the one you love.

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