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Natural blue diamonds are rare gemstones on Earth, second only to natural red diamonds. The weight of blue diamonds mined on Earth over the years is less than 600 carats. The top blue diamond presents a deep sea color or azure sky color, extremely charming. Of course, like the yellow and pink diamonds mentioned earlier, GIA also divides blue diamonds into several levels based on their color tones. The higher the color level of blue diamonds, the higher their value. Compared to other colored diamonds, blue diamonds are even rarer among the rarities.
Reasons for the formation of blue diamonds
Natural blue diamonds are typical type IIb diamonds (in addition to their rarity and rarity, their origin has always been an unknown mystery in geology).
The main reasons why blue diamonds form blue are as follows:
The first type is caused by boron, and the blue color of most blue diamonds is due to the inclusion of boron (B) during the diamond formation process. The more boron there is, the more intense the blue color becomes. Boron atoms have one less electron than carbon atoms, so when boron replaces carbon and enters the diamond lattice, a hole color center is formed. The electrons of boron atoms generate an impurity level (boron acceptor level) in the bandgap. The energy gap between the boron acceptor level and the diamond valence band (full band) is 0.4eV (with a larger width, not necessarily 0.4eV). Electrons can transition from the valence band (full band) to the boron acceptor level by absorbing 0.4eV energy (infrared region), absorbing red, yellow, and green light in visible light, while blue and purple light are not absorbed, Diamonds thus exhibit a beautiful blue or purple hue..
The second type is natural radiation, and due to the natural radiation induced coloration during the formation process, Type Ia and IIa blue diamonds usually exhibit varying shades of green and blue.
The third type of hydrogen (H) atom defect coloration, type Ia natural blue diamonds containing hydrogen (H) elements, exhibit colors ranging from gray purple to blue purple.
Another type is package coloring, usually blue diamonds with a strong gray tone.
Blue diamonds are different from all other colored diamonds in that they contain trace elements of boron during their formation and have conductivity properties.
Lab grown diamonds essentially involves artificially establishing certain conditions to transform non diamond structured carbon into diamond structured carbon. At present, there are two methods for cultivating diamonds: HPHT method and CVD method.
1. HPHT synthesis of blue diamonds: This method generally produces Ib type yellow brown diamonds. To obtain synthetic blue diamonds, nitrogen absorbers and boron need to be added to the growth chamber. After adding nitrogen absorbent, nearly colorless IIa type diamonds can be obtained, but the growth of this colorless diamond is very difficult. Moreover, colorless diamonds grown with nitrogen absorbers can detect very few individual nitrogen atoms. By introducing boron and removing most of the nitrogen with nitrogen absorbers, the diamonds can exhibit a very bright blue color.
The synthesized blue diamond has a high boron content, while the boron content in natural blue diamonds does not exceed 0.5ppm (ppm is one millionth). And the distribution of boron elements varies depending on the growth zone, with the highest content in the octahedral growth zone of diamond crystals and the lowest in the tetrahedral growth zone. This results in an uneven distribution of visible blue colors to the naked eye. In addition, when adding boron to the synthesis of blue diamonds, it is inevitable to add nitrogen, which can enhance the absorption of the blue region and increase the transmission of the yellow green region, ultimately leading to the synthesized blue diamonds having a green hue.
2. CVD synthesis of blue diamonds: It is a method of synthesizing diamonds by decomposing carbon containing gases at a low pressure equivalent to one tenth of atmospheric pressure. There are many methods for decomposing carbon containing gases into plasma, and the most commonly used is under high-temperature microwave conditions. CVD synthesis of blue diamonds also involves removing nitrogen from the raw materials and equipment during the synthesis process, and then adding boron, resulting in a gradual reduction in absorption from red to blue light, leading to the diamond appearing blue.
By changing the color in the later stage, some natural or Lab grown diamonds (cultivated diamonds )can also be processed into blue, mainly through HPHT treatment and irradiation treatment:
1. HPHT treatment of blue diamonds: High temperature and high pressure conditions in the laboratory provide sufficient isotropic pressure and potential energy for lattice defects in brown, brownish yellow, and brown diamonds. By artificially adjusting the temperature, pressure, and medium conditions, it can help improve the color level of diamonds or change their color.
For Ia type brown, brown yellow, and brown diamonds, due to the presence of chromogenic impurities such as nitrogen atoms and vacancies in their lattice, it is almost impossible to significantly increase their color level under existing technical conditions. Therefore, based on the existing plastic deformation of diamonds, through high-temperature and high-pressure reprocessing and further loading of their plastic deformation strength, the proliferation and slip of dislocations within the crystal are promoted, thereby achieving the goal of color modification.
IIa type brown, brownish yellow, and brown diamonds overcome their potential barriers and promote dislocation movement, recombination, and annihilation in diamonds under high temperature and pressure conditions, restoring them to their initial stable state before plastic deformation and maximizing their original colorless appearance.
It can be inferred that the HPHT treatment of blue diamonds is due to the disappearance of gray plastic defects within the diamond under HPHT conditions, resulting in the production of pure blue.
2. Radiation treatment of blue diamonds: Currently, the most commonly used methods for diamond irradiation color modification are neutron irradiation and electron irradiation. Neutron irradiation treatment mainly uses neutrons in nuclear reactors to bombard diamonds. As neutrons are not charged, they can easily reach the range of nuclear force when colliding with diamonds, which can cause nuclear reactions and easily make the diamonds appear colored as a whole. Electron irradiation of diamonds mainly occurs by accelerating electron bombardment, causing a series of elastic or inelastic collisions, generating color centers, and selectively absorbing a certain amount of visible light to ultimately produce color.
IIa type diamonds can knock carbon atoms out of the lattice through high-energy electron irradiation, producing a neutral hole called the GR1 color center. The zero phonon peak absorption peak of the GR1 color center is 740.9nm, and a wide absorption band is formed in the wavelength range of 430-412nm, accompanied by an absorption peak of GR2-8. The GR1 color center itself produces blue in diamonds, and when GR2~8 is strong, the diamond color is a greenish blue.
After irradiation, type Ia diamonds also produce GR1 color centers, which also have N3 color centers. N3 color centers absorb shortwave visible light, while GR1 color centers absorb longwave visible light. The strength of the two color centers determines whether the diamond appears blue or green. Generally, irradiated blue diamonds have much higher color saturation than natural blue diamonds.
