Diamonds have been one of the most beautiful gifts you can give to the people you love. Especially in romantic relationships. What if that diamond sparkles? When you present your fiancee’s engagement ring at a dimly lit restaurant or at dinner, what you want to hear isn’t “Hey look!! This diamond is radioactive!”. However, the “fluorescence” phenomenon in diamonds can be quite a nice thing if you understand what it is.
About one-third of a gem-quality diamond produces luminescence when exposed to UV light. In most cases, luminescence stops when the UV light source is removed. This phenomenon is known as fluorescence. In diamond, fluorescence is the purple, green, yellow, and red glow that occurs as a result of the excitation of boron molecules with ultraviolet light that transfers energy to the molecules. Fluorescence is a feature that can enhance the beauty of many diamonds, especially those with warmer (J K L M ) colors.
Fluorescence in Diamonds
According to the Gemological Institute of America, about 50% of gem diamonds contain enough fluorescence to be visible under special lighting conditions, such as exposure to ultraviolet (UV) light. Less than 10% of all gem-quality diamonds fluoresce enough to make a noticeable difference between diamond colors when viewed under incandescent (warm) light (low in the ultraviolet) and sunlight or fluorescent light (high in the ultraviolet).
Strong blue fluorescence in a D – H color diamond can cause the diamond to appear foggy or oily, but such cases are extremely rare. In fact, it is estimated that less than 2% of all diamonds on the market have such high levels of fluorescence that it reduces the gem’s luster. The owner of such a diamond may experience disappointment. However, this is not always the case, often the strong blue fluorescence will not negatively affect the diamond and will often improve the color appearance of a diamond.
Fluorescent diamond is a factor that needs to be evaluated individually. Fluorescence can fool us. It is important to be aware of the fluorescence and then see the diamond to determine whether it has a positive or negative effect on the diamond or to decide if it has no effect at all.
The color of a pale yellow (J K L M) diamond with strong blue fluorescence is usually white in color. I have seen a diamond that is graded Q (very light yellow) that appears white or bluish in daylight because it has a very bright blue fluorescence. These types of diamonds are sometimes called “premium priority diamonds”, but keep this in mind as just an “adjective”, these types of diamonds can be tried to be sold more expensive by the retailer, so pay attention to the price and certification when buying.
Many people who buy a diamond online tend to stay away from diamonds with any fluorescence as this is a factor that they cannot judge from the listing on the internet. However, many of the people we personally encounter when we have a jewelry store buy the diamonds even if they are fluorescent, after seeing the fluorescence that does not have a negative effect on the diamond. As a matter of fact, fluorescent diamonds can look very nice in the dark.
Many of the professionals and independent GIA Graduate Gemologists I have met so far in the Diamond industry gift or prefer diamonds that exhibit varying degrees of blue fluorescence to their spouses, or if they are females themselves; we as professionals would not purchase fluorescent diamonds for ourselves if fluorescence was a negative factor.
Diamonds with strong blue fluorescence often display a soft lavender-blue hue when the diamond is viewed from a side profile in direct sunlight (high UV). Although the lavender-blue color provided by the strong blue fluorescence is quite pleasant, it is quite difficult for most people to notice.
Fluorescence is the color response of money under UV light, where we used to see if money was fake. Some diamonds release energy when excited with UV light, scientifically this is called photoluminescence or cold glow. Diamonds are classified by the light intensity they emit under UV light. Diamond fluorescence levels are called like below:
- “None” in America “Nil” in Europe to those who did not react
- “Faint” in America “Slight” in Europe to those who react weakly
- “Medium” everywhere to the moderately responsive
- “Strong” everywhere to those who react strongly
- And “Very Strong” everywhere to those who react very strongly.
Studies show that approximately 30% of diamonds are fluorescent. Fluorescences are not always blue. Even though it is mostly blue, sometimes we can encounter yellow and very very rarely red, pink, green, and white fluorescence. The important thing when classifying here is not the color but the intensity. It can be in a faint stone with yellow fluorescence but in a strong stone. So, does fluorescence add or depreciate diamonds?
First of all, let’s remember that white light contains all colors from red to purple. Each color has a secondary color that complements it to white. The secondary color of red is turquoise, and the secondary color of blue is yellow. We have also seen this when adjusting the contrast settings of old televisions. While blue fluorescence creates a hazy oily appearance in white stones in the D – H color range, in light yellow stones in the I – M range, blue fluorescence gives a whiter appearance when viewed from the crown, as it contrasts with the color of the stone.
Yellow fluorescence causes a dull appearance in every stone. Therefore, in white stones, blue fluorescence has a bad effect on the stone and causes its price to decrease. In light yellow stones, it is even seen to increase its price because it causes a better appearance.
Factors Affecting Diamond’s Fluorescence
- Structural Rigidity
Rigidity is defined as the inability to deform under load. It has been empirically found that fluorescence is higher in molecules with rigid structures. Rigidity reduces the rate of the non-radiative slump and allows radiant simmering, in other words, fluorescence, to increase. (Free rotations around single bonds increase the radiant relaxation rate and decrease fluorescence.)
- Resonance Boundary Formula
The more the resonance boundary formula of the molecule can be written, the higher the fluorescence intensity. Resonance is the probability that π e-s can be everywhere. The greater this probability, the greater the crossovers.
The fluorescence of an aromatic compound containing acidic and basic substituents is generally pH-dependent. For example, aniline has only one resonance state, while aniline has many resonance states.
- Temperature and Solvent Effect
Increasing the frequency of collisions at high temperatures increases the probability of radiant stagnation and reduces fluorescence. Solvents containing heavy atoms or other solutes containing these atoms reduce fluorescence. For example, carbon tetrabromide is influencing. (Orbital spin interactions cause an increase in triplet formation rate and thus a decrease in fluorescence.)
- Electronic Passes
The more probable the electronic transitions, the higher the probability of fluorescence. The greater the π → π* transition, the greater the fluorescence. The fluorescence intensity at the n → π* transition is lower than at the π → π* transition.
As the number of cyclizations in the molecule increases, the fluorescence property increases.
- Existence of Heteroatoms
It increases the fluorescence feature. If rings containing heteroatoms are combined with a 2nd and 3rd aromatic ring, the fluorescence property increases. (The heteroatom must not break the ringing!)
- Chelate Formation
It increases the fluorescence feature. One of the frequently used chelators is 8-hydroxyquinoline (heteroatom in its structure) and it has fluorescence properties. The fluorescence properties of the chelates given by Zn+2 and Al+3 with 8-hydroxyquinoline are higher than that of the chelator itself.
The method based on the fluorescence phenomenon is called “Florimetry”. The fluorescence phenomenon is based on emission. To explain this, it is first necessary to define absorption. If a beam containing rays of various wavelengths is passed through a transparent medium, it is seen that the rays of some wavelengths disappear. This is called absorption of the beam.
By absorption, the radiant energy is transferred to the atoms, ions, or molecules of the substance. Ions, atoms, or molecules that have absorbed the beam energy become excited. The transition of excited atoms, ions, or molecules to lower energy levels by irradiating at certain wavelengths is called “Emission”.
Almost all fluorescence measuring instruments use dual-beam optics to compensate for fluctuations in the power supply. Fluorescence is from the sample in all directions (scattering is measured). The most appropriate fluorescence measurement is obtained from the beam propagating perpendicular to the excitation beam, this is the most efficient beam. In other respects, scattering from solution and cell wall can cause large errors in intensity measurement. Therefore, in both devices, the beam arrives at the detector at an angle of 90ᵒ.
Gemological Analysis of Color Change and Fluorescence in Diamonds
Diamond, which was formed in nature over a very long time, such as three billion years, is the most precious stone among precious stones and has the highest material value. What makes a diamond so precious is both its time to form and its rarity. In this direction, diamonds with unusual colors as well as fluorescence diamonds are much more expensive because they are less common. Examples of colors are pink, blue, orange, green, and red. While shades of white and yellow are common, shades of blue or red are rare. Therefore, a blue or red diamond is much more expensive than a white or yellow diamond.
Just like every person is unique, every diamond in nature has its own characteristics and structure. These differences become a source of curiosity for researchers and open the door to new investigations. Today, the most current issue is the improvement of diamonds in the laboratory environment or the re-creation of a diamond. This formation of stones, which we call synthetic, is becoming more common day by day and is starting to enter our lives. In the period when the production of synthetic diamonds started, the manufacturers stated that the jewelers were worried for nothing and that synthetic stones were not available in the market to a great extent.
However, the entry of synthetic stones into the market is increasing day by day and the threat it poses is growing. It is very difficult for a consumer to understand whether the stone is synthetic, color-treated, or natural. Because this difference can only be revealed by an expert gemologist by examining it with gemological devices. A color-treated stone may not be distinguishable from its natural with the naked eye. However, there are (big) material and moral differences. Stones that have been cured by treatment are much cheaper, and their moral value cannot be compared with the natural ones because they are touched by human hands.
Diamonds can have different and precious colors. Each stone has its own unique characteristics. Just like every person’s fingerprint is unique, every diamond has its own unique values. Even though they look exactly the same at first glance, there is definitely a tonal difference in their color, a difference in fluorescent colors, or a difference in the size/ratio of the stain inside.
Diamond consists of pure carbon. However, sometimes different substances can enter into it during formation, which causes color changes. A diamond composed entirely of pure carbon is colorless. A diamond with a boron atom is in shades of blue, gray, or bluish-gray. The boron ion that gives color to the stone is limited to a few per million carbons. It has been observed that some stones with crystalline structure disorders have colorations such as pink purple. A nitrogen-containing diamond can be yellow, brown, brownish yellow, or orange. The way nitrogen is found (such as being scattered or grouped) also causes color differences.
Fluorescence and Color Change in Diamonds
Because diamond is a good insulator, it has a high energy range. It passes all wavelengths of white light and thus becomes transparent and colorless. However, the fact that some impurities in it create various additional energy levels causes absorptions. For example, sometimes all wavelengths higher than nitrogen and boron in diamond are absorbed and nitrogen and boron fill the band gaps. This causes discoloration in diamonds. Most diamonds contain nitrogen. Sometimes it can be a very trace amount and sometimes it can be intense.
In essence, diamond consists of pure carbon. Each carbon atom in the crystal structure of the diamond is bonded equidistantly with the other four carbon atoms by covalent bonds. For this reason, it is very strong, and its crystal structure is so regular and ordered that it is unique. It has been proven that stones with different colors give different peak values. The reason why different colors give different values is due to the different atoms and their arrangement. By looking at these values, it is possible to comment on whether the stone is natural or artificial. At the same time, it can be understood whether he has been treated or not.
It is an expected situation for white stones to have a peak value at 2300-2800 cm-1 values. However, unlike a stone, for example, a peak value at a value of 3000-odd cm-1 can be seen. It is thought that this is due to the light pink tone in it. According to the results of fluorescent color and peak values, it can be understood that the stones are both natural and untreated. The fact that the color of a diamond is yellow suggests that it contains nitrogen, and the peak values are proven to be the peak values that should be in a yellow stone containing typical nitrogen.
Examining a precious metal such as a diamond in detail is as enjoyable as it is an important job. Diamonds of different colors obtained for the examination can be examined individually in the laboratory environment, both physically and chemically. With the help of a balance, their weight, color card, and scale and colors, their clarity with the help of a loop and microscope, fluorescent colors with the help of ultraviolet light, and peak values that give information about their chemical properties with the FT-IR device can be analyzed. Although the peak values given by the FT-IR device do not show a direct result, it is 100% reliable to comment on the stone.
The FT-IR (Fourier Transform Infrared Spectroscopy) instrument is a type of vibration spectroscopy; infrared rays are absorbed by the vibrational movements of the molecule. With this method, molecular bond characterization is performed; functional groups in the structure of solid, liquid, gaseous, or solution organic compounds, whether the two compounds are the same, the state of the bonds in the structure, the bonding sites, and whether the structure is aromatic or aliphatic can be determined. Speed, sensitivity, and accuracy are extremely high in FT-IR spectrometers.
There are two axes that make up an infrared spectrum. One is the horizontal axis (frequency or wavenumber) and the other is the vertical axis (absorbance). Wave numbers on the horizontal axis will be examined while color examinations are being made. The wavenumber is the number of vibrations of the wave per centimeter of the beam and is expressed in units of cm-1. Cm-1 represents the number of waves per unit length.
The absorbance value on the vertical axis is defined as absorption. The values on the vertical axis may vary in direct proportion to the size of the stone. The vertical spacing can be increased for small stones so that the graph can be displayed more clearly. With these methods, it is possible to check the color values of the stone, whether it has been treated or not, and it also contributes significantly to making the synthetic-natural distinction.
The color change in diamonds takes place during formation as in all-natural stones. Color differences are observed in all diamonds containing atoms other than carbon. Each color has a different value. For example, it can be said that a stone with a peak at 3100 cm-1 has a yellow or brown tone, stones with a peak at around 2300 cm-1, 2500 cm-1 are white and the value of 3600 cm-1 is due to its pink color.
Natural color differences are a very important criterion that makes the diamond even more valuable. Artificial coloring, on the other hand, is the treatment made with the help of machines by touching human hands in the laboratory environment. HPHT (High-pressure, high-temperature), which means high temperature and high pressure, is the most frequently used method. HPHT treatment started to be applied in 1999. With this method, desired changes can be made by applying temperature and pressure to the stone.
Fluorescence Measuring Devices
In wavelength selection, filters are used as in photometers. These filters are used to limit the wavelengths of excitation and emitted rays.
Unlike fluorimeters, this type of device has 2 monochromators. These are excitation and emission monochromators. In this way, both excitation and emission spectra are obtained.
Monochromator: (Wavelength Selectors) It is the mechanism used to obtain monochromatic light (light with a single wavelength) from polychromatic light (light of more than one wavelength) coming from the light source.
Excitation Monochromator: It is used to limit the excitation radiation. While the light of the wavelength required for fluorescence passes, rays of other wavelengths are retained.
Emission Monochromator: It is used to separate the radiation from the sample into its components.
The wavelength of the emission (fluorescence) beam will usually be longer than the wavelength of the excitation beam, that is, it will have lower energy. The fluorescence intensity is very low compared to the absorption intensity. Fluorescence is very weak radiation and its intensity is around 1% of the light intensity that stimulates it.
For this reason, a beam intensifier is used in the device in order to compare the fluorescence beam that is emitted on the sample with the beam that is emitted. However, since the ratio of fluorescence intensity to excitation beam intensity is not known at the beginning, fluorescence measuring devices read the result relative to a standard substance. Therefore, the value read is unitless (relative frequency intensity). Standard materials used are quinine and fluorescein.