Features of main treatment methods

Removing inclusions by laser drilling

   Laser drilling has been used since 1970s to improve diamond clarity. This method is most often applied to those diamonds containing relatively large dark inclusions.

   The method implies drilling a narrow channel (with a diameter of up to 25 micrometers) in a diamond by means of a laser. The channel starts at the diamond surface and ends inside an inclusion. Then, an acid is fed into the channel to dissolve the inclusion.

   The following dark-color inclusions are most often removed by the laser drilling method:

  • sulfides (pyrite, pentlandite, and pyrrotine);
  • chromite grains;
  • crystals and shapeless clusters of epigenetic graphite;
  • magnetite, rutile, etc.

Fracture filling

   This method has been used since 1987 to improve diamond clarity. Though fracture filling processes implemented by different companies have some peculiarities, the common feature of all these processes is placing the diamond to treat into a filling material heated to 500- 600C at a high pressure or in vacuum.

   The filling material is usually a special glass characterized by:

  • high concentration of heavy elements (Pb, Bi, Br), as well as of B, Cl, and O;
  • high refractive index for the visible region, almost equal to that of diamond (2.4);
  • relative colorlessness;
  • relatively low softening temperature (from 340 to 390C).

Coating

   This method is aimed at improving diamond color and implies coating either the whole surface of the treated diamond or some portions of the surface (for example, in the girdle region) with a narrow film. The composition of the film may vary. Coatings based on compounds of fluorine and silicon as well as special dyes can be found. These coatings are usually colored so as to make the diamond fancy, blue and pink being the most popular fancy colors. Another type of coating is used to remove yellowish colour of a diamond. The material of such a coating absorbs long-wavelength visible light.

   Processing a coated diamond with a boiling acid or a medium-hardness abrasive (such as quartz) removes the coating completely or partially (scratches appear on the coating). Heating a treated diamond may change the optical properties of its coating. This treatment method is reversible.


Irradiation

   This method is used to improve diamond color. Usually, yellowish diamonds are processed to acquire fancy blue, green, brown, black (greenish black), orange, or canary yellow color.

   This treatment method implies irradiating a diamond with:

  • salts of radium or other transuranium elements;
  • protons, alpha particles, or other heavy particles, accelerated in cyclotrons or linear accelerators;
  • neutrons produced in nuclear reactors;
  • high-energy electrons.

   Any of the above irradiation methods causes GR1 centers (neutral vacancies) to appear in the irradiated diamond. These centers are responsible for the appearance of blue-green coloration in the diamond.

   Irradiation with salts of radium or other transuranium elements was first applied to diamonds in 1904. As a result of such irradiation, the treated diamond acquires green coloration only in its surface layer. This coloration is often non-uniform: green spots are observed on the diamond surface. When viewing such a stone through its table, an "umbrella effect" can be often observed, which is due to the non-uniform coloration of pavilion facets. The diamonds treated in such a way become radioactive sources with a power that is sometimes enough to consider these diamonds harmful to health.

   Irradiating diamonds with heavy particles accelerated in cyclotrons or linear accelerators yields the same results as the radium salt irradiation. The difference is the absence of green spots.

   High-energy electron irradiation and neutron irradiation are now the most preferable and most common irradiation methods. The mean free path of these particles in a diamond exceeds 10 mm, which causes the diamond to become uniformly colored. In most cases, the diamonds irradiated in such a way do not become radioactive.

   The irradiation-based diamond treatment can be revealed by analyzing the features of optical absorption spectra or luminescence spectra. This analysis requires the use of special equipment.


Irradiation followed by annealing

   Annealing is widely used to change the irradiation-induced color of a diamond. Annealing destroys irradiation-induced GR1 centers in the diamond and creates new color centers. The type and concentration of the new centers depend on the physical type of the diamond and the annealing temperature. As the annealing temperature increases, the color of the annealed diamond usually changes in accordance with the following sequence: blue (cyan), green, brown, yellow, and, finally, the original color of the diamond (before the irradiation). Annealing can be stopped at any of these stages. In general, the annealing method allows one to make the diamond color only darker than that before the irradiation, although lighter than that after the irradiation.

   The annealing-based diamond treatment can be revealed by analyzing the features of optical absorption spectra or luminescence spectra. This analysis requires the use of special equipment.


High Pressure High Temperature (HPHT) method

   This method is aimed at improving the color characteristics of a diamond. At the same time, its clarity characteristics may uncontrollably change.

   The HPHT method is used to treat diamonds of different physical types (Table 1), which have yellow, yellow-brown, or brown color, undesirable for jewelry purposes. The treatment of type IIa diamonds in most cases yields the best results: the color characteristics of the diamonds considerably improve. When treating type Ia and type Ib diamonds, they often retain some yellowish colour (in most cases, this tone is much lighter than that before the treatment).

Table 1. Diamonds treatment by the High Pressure High Temperature method

Physical type of diamond

Original color

Treatment-induced color

IIa

brown

colorless, color of "cape series"

IIa

brown

pink

IIb

brown

blue

Ia

yellow, brown

yellow-green, brown-yellow

Ia

colorless

canary yellow

   The HPHT method implies subjecting diamonds to a high pressure and a high temperature in an appliance used for producing synthetic diamonds. In type IIa and type Ia diamonds, the HPHT process removes plastic strains, which are responsible for the undesired color, while in type Ib diamonds separate nitrogen atoms, which cause the yellowish colour, start aggregating.

   Nowadays, the HPHT diamond treatment is most difficult to reveal. For this purpose, spectroscopic techniques are commonly used. After a diamond is HPHT-processed, the label "GE - POL" is laser-engraved on its girdle.