diamond treatments - gemological background

The four C's determine the value of a diamond and, as everyone knows, changing one or more of the four C's can increase the value of a diamond substantially. The higher the clarity and the better the color, the more expensive a stone is. This has inevitably led to a variety of methods that are used to "improve" a diamond's natural clarity and color. Improving on nature is by no means a modern invention. From the ancient alchemists, there has been a constant struggle between the inventors and detectors of diamond treatments. Gemological laboratories such as International Diamond Laboratories work incessantly in ensuring no treatment, whether temporary or permanent, can  pass undetected.

Existing treatments can be divided into two broad groups: methods to enhance the clarity of a diamond and techniques to change the color of a diamond.

clarity enhancing treatments

fracture filling
Fracture filling is the process of filling natural cracks or cleavages. A cleavage can be naturally present in the rough stone or develop during the processing of the diamond. Since a large crack has a significant negative impact on the clarity, this will seriously decrease the value of the diamond. The aim of the cleavage (or fracture) filling is to decrease the visibility of such a crack. The process involves the use of specially-formulated glasses with a refractive index approximating that of diamond.
Diamonds that have been submitted to such a treatment can be identified through a number of visual inspections. These can be detected with a microscope, as well as with a loupe 10X.
Parts of the composition of the glass filling can be detected easily by X ray fluorescence, giving a positive and clear identification of presence of a filling. 

laser drilling
A black inclusion of crystal can influence the color of a diamond and this inclusion can be removed by a laser drilling. In this treatment, a narrow channel is created by locally damaging the diamond with one or several lasers. Once an entry point has been created, a mixture of boiling acid is introduced under pressure and the inclusion subsequently bleached.
The ‘KM treatment' is a new laser drilling process that removes black inclusions from a stone subsequently improving the diamond's clarity grade. ‘KM' are the initials of a term in Hebrew (Kiduah Mayuhad) meaning ‘special boring'. The process involves creating a passage from the surface of the stone to the inclusion - the passage looks more like a cleavage than a drilled channel. 


color enhancement treatments

irradiated diamonds
Over a century ago diamonds underwent a form of radiation to change their color. Sir William Crookes, a gem connoisseur as well as a chemist and physicist, conducted a series of experiments using radium salts which he found turned diamonds a dark green color. The process was by no means a total success, not only was the color localized in blotchy patches, it also only existed on the surface of the stone.
Nowadays diamonds are irradiated in one of four ways: proton and deuteron bombardment via cyclotrons; gamma ray bombardment via exposure to cobalt-60; neutron bombardment via the piles of nuclear reactors; and electron bombardment via Van de Graaff generators or Linac's.
These high-energy particles physically alter the diamond's crystal lattice, knocking carbon atoms out of place and producing a point defect, also called a color centre. Irradiated diamonds are all some shade of green, black, or blue after treatment, but most are annealed to further modify their color into bright shades of yellow, orange, brown, or pink. The annealing process increases the mobility of individual carbon atoms, allowing some of the lattice defects created during irradiation to be corrected. The final color is dependent on the diamond's composition and the temperature and length of annealing.
The two most common irradiation methods are neutron and electron bombardment. The former treatment produces a green to black color that penetrates the whole stone, while the latter treatment produces a blue, blue-green, or green color that only penetrates about a few micrometers to millimeter deep, depending on the energy of the impinging electrons.

hpht treated diamonds
High Pressure/High Temperature (HPHT) treatment can improve the color of certain types of brown stones and thus increase their value. In the mid-90s General Electric (GE) developed a process that mimics certain aspects of the stone's creation in Earth. The stone is heated up to a very high temperature (T > 2000°C) under a high, stabilizing pressure (P > 7 Gpa). Faults in the crystal lattice are thus rearranged causing an alteration in the stone's color. Because the conditions are very similar to the stone's original formation, detection of this process is can only be carried out by a well-equipped lab.
To begin with, the only stones that can successfully be rendered colorless by HPHT treatment are those known as type IIa diamonds - this means diamonds with low nitrogen contents or type IaB, in which the nitrogen is present in the more stable aggregation form B, four nitrogen atoms surrounding a vacancy.
Type IIa diamonds are extremely rare (it is estimated that just 1% of all mined diamonds are in the type IIa category) and treatable type IaB diamonds are even rarer. Type IIa diamonds contain a minute number of substitute nitrogen atoms which makes them malleable for this color improving process. The preponderant brown (sometimes pink) color of these stones is caused by the distortion of the crystal lattice and growth structure of the diamond due to the geological forces deep under the Earth's surface. During HPHT treatment the defects in the crystal lattice are rearranged so that a colorless or near-colorless diamond emerges out of a brown type IIa diamond.

coated diamonds
The coating on polished diamonds refers to a thin artificial layer that alters the color of the diamond. The coating can either be of the desired color or can cause a colored interference-effect.
Coating can be applied to the whole of a diamond's surface or to a narrow zone near the girdle. A well-applied coating can change the color by one color grade.
Most coatings consist of fluoride, silicon oxide (SiO2), aluminum oxide (AI2O3) or titanium oxide (Ti02). Some coatings can be removed through boiling in sulfuric acid or hydrofluoric acid.
Recently, pink coated diamonds have been submitted to diamond grading labs.