Fluorescence – Diamond’s Hidden Beauty
As if diamonds aren’t special enough, some diamonds do something remarkable – they glow when exposed to ultraviolet (UV) light, also known as “black light”. This property, luminescence, causes a soft glow to emit from the diamond, causing some diamonds to be luminous. In gem-quality diamonds, this is a relatively rare phenomenon and a property that can give a diamond an element of interest and wonder.
Knowing more about the technical details of fluorescence helps to better appreciate this phenomenon in some extraordinary diamonds.
The Science
Scientifically speaking, luminescence is a general term to describe the process in which a material (diamond) absorbs energy other than heat (light) from an external source (the sun or a UV lamp) and re-emits that energy in the form of visible light (glow). Luminescence is then sub-categorized by what energy source is applied, or how the material reacts after the energy is applied. It is encountered in everyday life more often than is recognized.
For example, chemiluminescence is the emission of light after a chemical reaction, such as glow sticks at a party, or chemical reactions used to detect blood by forensic scientists. Electroluminescence is the result of an electric current passed through a substance, like the backlight on a liquid crystal display (LCD) watch or clock.
Electroluminescence of a backlight watch face. Image @ GSI
Diamonds exhibit photoluminescence which is an emission of light as a result of photons (electromagnetic radiation such as radio waves, microwaves, infrared, light, ultraviolet, X-rays, and gamma rays) being applied to the material.
Photoluminescence is then subdivided as fluorescence, or phosphorescence according to how the material (diamond) reacts. Fluorescence is the glow that occurs while the energy source is applied and may only last a few nanoseconds (undetectable by the human eye) after the energy source is shut off Phosphorescence is the glow that is observed after the energy source is shut off and may last from milliseconds (detectable by the human eye from about 80 milliseconds) to a few hours.
The electromagnetic spectrum that can cause photoluminescence. Image @ defendershield.com
Fluorescent materials produce light instantly when the atoms inside them absorb energy and become "excited." When the atoms return to normal, in as little as a hundred-thousandth of a second, they give out the energy that excited them as tiny particles of light called photons, and this is the glow we see with our eyes. Phosphorescent materials work in much the same way as fluorescent ones, except that there is a delay between them absorbing energy and giving out light.
Diamonds are pure carbon crystallized in the cubic system. Diamonds may contain tiny amounts of elements such as nitrogen, boron, or hydrogen as part of their formation process. When these trace elements are present in a diamond’s atomic structure, energy from ultraviolet sources can cause the diamond to fluoresce. When diamonds are exposed to long-wave (365 nm) or short-wave (254 nm) ultraviolet light (called LWUV or SWUV respectively), a diamond that can fluoresce will absorb the higher energy UV light and jump up to higher energy states. Following the laws of physics, these excited electrons always seek to return to a stable state. They do this by getting rid of excess energy, releasing it in the form of light that we can see with our eyes as that mesmerizing glow.
A diamond in daylight equivalent light, and then showing fluorescence under long-wave ultraviolet light (LWUV) at 365 nm. Video @ GSI
Fluorescence in Diamonds
Fluorescence is only seen in about 25% to 30% of natural, near-colorless (D-Z) diamonds. Phosphorescence is even more uncommon.
Diamonds are evaluated and valued by the well-known 4 Cs, but fluorescence is also an important factor that can set a diamond apart from one with a comparable cut, color, clarity, or carat weight. Fluorescence is evaluated on a scale from “None” to “Very Strong”. Most gemological laboratories start reporting fluorescence at “Faint” as that is the point at which it is visible under a controlled laboratory environment, although “Faint” fluorescence is not easily observed in standard real-world lighting conditions.
A fluorescence assessment scale. Image @ GSI
Fluorescence is observed in about 25% to 30% of diamonds in any intensity. However, the stronger the intensity, the rarer fluorescence is. Only 2% of GSI graded diamonds have fluorescence in the strong or very strong range. This makes a diamond with strong or very strong fluorescence pretty special!
99% of the time, when they fluoresce, diamonds fluoresce blue. Blue fluorescence in diamonds is caused by nitrogen, which is the most common color center in diamond (which is the same atom that causes diamonds to appear different shades of yellow). Sometimes, if strong enough, the blue fluorescence can balance out the yellow body color of the diamond and help its appearance.
The other rare 1% (or less) of the time, diamonds can fluoresce other colors like yellow, white, or orange. The fluorescence color is determined by the chemical composition and structure of the diamond’s lattice, so there can be many causes of fluorescence in diamonds, but the conditions at the time the diamond was formed have to be “just right”.
Fluorescence Causes
Blue fluorescence requires nitrogen to aggregate (come together) in groups of three, surrounding a vacancy (a spot where a carbon atom would be but is empty). Bringing nitrogen atoms together in a diamond crystal takes a lot of time, heat, and pressure, (millions to billions of years under the Earth’s surface). For this reason, the presence of blue fluorescence is considered an indicator of natural versus laboratory-grown origin, as laboratory-grown diamonds are formed over weeks, and do not have the chance or conditions for nitrogen to aggregate. But, using the presence of blue fluorescence in diamonds must be applied with some caution as some laboratory-grown diamonds can also fluoresce blue, although the cause of the color and intensity are different. It is always important to send a diamond to a gemological laboratory like GSI, that specializes in natural versus laboratory-grown diamond detection
To make things more interesting, other types of nitrogen aggregates in diamonds can actually quench the N3 fluorescence, so not all diamonds with N3 centers will fluoresce. This also dispels the notion that diamonds with no fluorescence are “purer”. Boron, another color center in diamond can cause blue fluorescence as well as a blue phosphorescence.
In the super-rare cases of non-blue fluorescence in near-colorless diamonds, the cause of the color of fluorescence can be caused by several nitrogen-related centers. Yellow is caused by a lens-shaped cluster of nitrogen atoms called a “platelet”. Single nitrogen can cause orange fluorescence. Other color centers associated with nitrogen and
A diamond with yellow fluorescence. Image @ GSI
vacancies (H3 and H4 centers) can cause greenish fluorescence. Nitrogen content, nitrogen aggregation state, diamond structure, can all play a role in the color, intensity, and appearance of the fluorescence. Even more interesting, and to add to the complexity, a single diamond can contain more than one kind of optical center that causes fluorescence.
Because nature is perfectly imperfect, diamonds can even form with fluorescent zones that follow their crystal growth.
Note the zoning of the fluorescence in the natural diamonds. On the leftmost image, note the yellow zone within the blue bodycolor of the diamond. Image @ GSI
Non-blue fluorescence is much more common in fancy color diamonds because they have a higher concentration of color centers or mechanisms that cause both their color and influence their fluorescence. For example, if the element boron is in the crystal lattice of diamond, it can cause red fluorescence, which is what occurs with the Hope diamond.
The Hope Diamond and it’s famous red fluorescence caused by boron impurities.Image @ Smithsonian Institution
Observing fluorescence in diamonds depends on the intensity and wavelength of UV light used to excite it. In gemology, the LWUV source to examine diamond fluorescence is 365 nm at a distance of about 3 inches using an area light source. However, many fluorescent penlights on the market are 405 nm and are a more direct source of light than the cabinet bulbs used by gemologists. This may account for some discrepancies between observations; shining a penlight on a diamond during a sales presentation may make the fluorescence seem more intense than it may be in standard lighting.
In most lighting environments, diamonds do not show fluorescence readily, because most light sources do not have a strong UV component. This makes fluorescence like a secret property of diamonds – you only see it in UV light. Diamonds with strong or very strong fluorescence will more likely show fluorescence in sunlight that has a strong UV component, but most diamonds will only show fluorescence under “black light”.
Left: A diamond showing fluorescence in sunlight. Image @ Pricescope.com. Right: Hands at nail salon with fluorescent diamonds in the rings. Image @ http://boards.weddingbee.com
Fluorescence Perception
The jewelry industry has two perceptions of fluorescence in diamond:
- Blue fluorescence counteracts the yellow in diamonds and makes them appear more colorless, so it is a benefit.
- Blue fluorescence can make a diamond appear hazy or oily, and detracts from the beauty of the diamond.
Published studies have shown that fluorescence has no negative impact on a diamond’s beauty. In the very rare cases, the strength of fluorescence of the diamond and lighting environments are conducive, a diamond may show a haziness or cloudiness when exposed to UV light. These diamonds are called “overblue” and are very rare.
Diamonds with medium to very strong fluorescence may have a price break at the wholesale level, but this isn’t related to the visual impact of the fluorescence, but rather, relates to industry pricing from the past based on the market trends of fluorescent diamonds as far back as the 1970s.
Fluorescence in Jewelry
Jewelry designers and bespoke brands use diamond fluorescence in their jewelry designs to highlight the secret inner glow of some diamonds.
Jared’s new “Brilliance WithinTM” collection is designed to inspire and empower every woman to celebrate her inner strengths and shine from within, with the tag line “honor your inner light”.
A few products from the virtual reality boutique of Jared Jewelry, giving viewers immersive experience; featuring the stunning Brilliance Within™ collection. Image @ jaredjewelry
A few products from the virtual reality boutique of Jared Jewelry, giving viewers immersive experience; featuring the stunning Brilliance Within™ collection. Image @ jaredjewelry