There are two types of treatments: treated natural diamonds, and synthetic
diamonds.
Treated natural diamonds
The aim of treating natural diamonds is to artificially improve their quality
by changing one or several of their characteristics, thereby increasing their
value. Such treatments generally target color and purity.
For color, treatments can help to resolve problems related to quantity and
uniformity: if a jeweler needs a hundred or so identically colored diamonds
to set in a pavé, treating an entire lot of diamonds will enable him to
obtain a more uniform color.
The main types of treatments are presented hereunder. This list is as complete
as possible however this area is still the subject of much research and
development.
a. Modification of purity
Crack filling : process of filling natural cracks or cleavages with
substances that have a refractive index similar to that of diamonds. In this
way, the light is less refracted than it would be if the cleavage were filled
with air - and therefore much less visible. Laboratories do not issue
certificates for diamonds that have undergone this type of treatment.
Laser drilling : this process aims to lighten or completely eliminate dark
inclusions. A tiny canal is laser-drilled and high pressure boiling acid is
then injected in the hole. Laser drills are mentioned on certificates.
b. Modification of color
Heat treatment or annealing : this technique produces black diamonds. Low
quality diamonds are exposed to very high temperatures (around 1000°C) for
several hours, until graphite residues appear in the heart of the crystal,
making it dark and opaque or even uniformly black. Laboratories do not
deliver certificates for these treated diamonds.
Irradiation : exposure of a diamond to a source of radiation will change
the position of atoms within the crystal lattice, which will cause its color
to change. Green, blue and black diamonds can be created using this
technique. Laboratory certificates mention "treated".
Irradiation + heat treatment : after a first transformation via irradiation,
the color can be changed again by exposing the diamond to temperatures of
around 800°C. This will again re-position atoms within the crystal lattice,
and atomic defects can be repaired or removed. The diamond will absorb light
differently and will thus take on another color. As a general rule, brown,
yellow orange, blue, pink and purple diamonds can be obtained with this
treatment. Laboratory certificates mention "treated".
HPHT (high pressure, high temperature) : this process subjects the diamond
to a combination of very high pressure (HP) and very high temperature (HT),
similar to the conditions in which diamonds are formed in nature. This will
cause imperfections in the crystal lattice to be "repaired",
provoking a change in its color. Depending on the type of stone that is
treated, the HPHT process can make a stone more colorless, or can produce
blue, yellow orange, yellow green, pink or brown diamonds. Certificates
mention "HPHT".
Coating : with this process, a thin layer of colored substance is deposited
on the diamond in order to change its original color. Coating is falling into
disuse and laboratories do not issue certificates for these types of treated
diamonds.
Color
Heat treatment
Irradiation
Irradiation
+ heat treatment
HPHT treatment
Blue
x
x
x
Green
x
Yellow orange
x
x
Yellow green
x
Pink
x
x
Brown
x
Black
x
x
Synthetic diamonds
Synthetic diamonds are created artificially from carbon. This process barely
requires three weeks, compared to tens of thousands of years for natural
diamonds. Only laboratories possessing leading-edge technology are able to
detect these diamonds, which have exactly the same physical and chemical
properties as natural diamonds. Laboratories do not deliver certificates for
these stones.
Two different techniques are currently used to produce synthetic diamonds :
HPHT (high pressure high temperature) : this is the best-known technique.
HPHT subjects carbon atoms to very high temperatures and pressures,
re-creating the geological conditions in which diamonds crystallize. Diamonds
created using this technique are extremely similar to the "real
thing" and one of the only ways to detect them is to use special
equipment that can reveal the growth lines of the crystal. These lines are
younger in synthetic diamonds than in natural diamonds and this can be seen.
HPHT processes produce yellow and orange diamonds that can be quite large
(0.5 to 3 carats).
- CVD (chemical vapor deposition) : this technique was
developed in the late 1960's and involves the propulsion of CO2 into plasma.
The diamonds created with that process are low nitrogen type IIa crystals.
Their color is usually brown, but also sometimes colorless, faint pink, blue
or black.
