Colors of Lab Grown Diamonds and Its Causes
Many buyers are unaware that diamond colors can be completely different from the traditional white stone and can actually vary between red, blue, green, pink, yellow, or brown. Natural colored diamonds are very rare, so they are not often found in jewelry stores. Since the makeup of these stones’ color combinations is unique, each stone is evaluated at an auction, where prices can reach millions of dollars per carat.
With the development of production technologies such as HPHT and CVD, the market for laboratory-grown diamonds has changed drastically - the entire spectrum of diamond colors became easily accessible. As a result, the cost of these stones are more reasonable, no longer valued at millions of dollars per carat, but now at the level of grown colorless D-color stones (+ 10-20% from Rapaport).
Without the precedent set by their natural “older brothers,” the reasons behind the different colors of lab-grown diamonds would be moot. The color variation in diamonds was studied in the 20th century using natural stones. Their findings eventually led to the opportunity to create lab-grown stones, by replicating the control and environment that nature created over millions of years. Thus, the scientific reasons for the appearance of color in grown and natural diamonds are the same. The only difference is that natural diamonds were formed deep in the Earth's mantle and have been in such conditions for billions of years, and lab-grown stones are man made in 2 weeks under extreme temperature and pressures - conditions close to natural, but artificial. The color of any diamond is determined by the presence or absence of defect-impurity centers in the crystal lattice of a gem. An ideal colorless diamond consists of carbon atoms, but as soon as atoms of another element are incorporated into its structure or there are violations in the arrangement of atoms, a certain color begins to appear in the diamond.
In 1934, Robertson, Fox, and Martin created a physical classification of diamonds, which was refined by many other scientists, and is widely used to this day. The classification is based on the presence of the main impurities in diamond crystals, nitrogen and boron. These impurities most often determine the color of the diamond. If boron is incorporated into the diamond structure only in the form of single atoms, then nitrogen forms more than 15 different defect centers in the diamond structure (single atoms, paired atoms, combinations of several nitrogen atoms with vacancies, etc.). The presence of nitrogen and boron impurities in diamonds is determined mainly using the method of FTIR spectroscopy.
Let's explain the physical classification of diamonds in more detail. The classification standards are applicable in equal rights to both natural and grown diamonds. As you can see in the picture, initially all diamonds were divided into 2 types according to the presence of nitrogen impurity:
- I — with nitrogen
- II — without nitrogen.
Type I was divided in more detail than Type II (since there are many different nitrogen defects in diamond):
- I a — diamonds with aggregated nitrogen atoms in the structure
- subtype IaA — diamonds with defects A (a pair of nitrogen atoms),
- subtype IaB — diamonds with B1 defects (four nitrogen atoms + vacancy),
- subtype IaAB — mixed type, diamonds with A and B1 defects;
- I b — diamonds with single nitrogen atoms in the structure (C defects), yellow;
Type II was then split into 2 subtypes:
- II a — pure diamonds, colorless or brownish;
- II b — diamonds with single boron atoms in the structure, blue.
Laboratory-grown Diamond Colors
Moving from complex terminology to a simpler one suitable for use in the market of lab-grown diamonds, 95% of lab-grown diamonds fall into three types according to their physical characteristics: IIa-colorless, IIb-blue, and Ib-yellow. It is important to note, naturally occurring diamonds of all three types are isolated cases, where 95% of natural diamonds are type Ia. This is the main difference between natural and grown diamonds and is often used to identify them.
- Yellow - due to the entry of single nitrogen atoms into the structure of
diamond, type I b.
HPHT technology - common product, the color is well controlled. Nitrogen is captured from the atmosphere or controlled by the composition of the catalyst metal alloy.CVD technology - not often produced, more difficult to control color. Nitrogen is part of the gas mixture.
- Green - due to the occurrence of vacancies in the structure of the diamond.
Both technologies - as a rule, irradiation of type Ib or IIa diamonds with a beam of fast electrons (with an energy of 1-3 MeV).
- Pink and Red - due to nitrogen vacancy NV centers.
HPHT technology - a) low-saturated yellow diamonds of type Ib (with single nitrogen atoms) are grown; b) irradiate them with a beam of fast electrons (with an energy of 1-3 MeV) in charged particle accelerators or reactors (vacancies are formed); c) they are annealed in furnaces at Т=800-1200оС (NV centers are formed).CVD technology - either similar to HPHT technology, or NV centers are formed directly during the growth process.
- Blue - due to the entry of single boron atoms into the structure of the
diamond, type IIb.
HPHT technology - common product, the color is well controlled. Boron (fraction of %) is added to the composition of the metal catalyst alloy.CVD technology - not often produced, more difficult to control color. Boron is added to the gas mixture supplied to the growth chamber.
- Colorless - no impurities, type II a.
HPHT technology - common product, the color is well controlled. The absence of impurities is achieved by the presence of "getters" (gas absorbers) in the composition of the metal-catalyst alloy.CVD technology - common product, the color is less controlled, often a light brownish tint.
- Brown - deformation disturbances or nickel-nitrogen centers.
HPHT Technology - rarely produced, not in demand. Crystals with a high concentration of nitrogen and an impurity of nickel in the structure. Depends on the composition of the catalyst metal alloy.CVD technology - often found when the quality of the synthesis process is poor. Dislocations and structural disturbances are formed.
The color saturation of a laboratory-grown diamond depends on the concentration of defect-impurity centers. For diamonds, due to the peculiarities of the atom arrangement in the structure, a very low concentration of impurities is sufficient for it to begin to acquire color. The concentration of impurity elements in a diamond is measured in ppm (parts per million) or even ppb (parts per billion). For example, a yellow diamond with a nitrogen concentration of 1 ppm will have one nitrogen atom in the structure per million carbon atoms -such a small fraction of impure atoms already creates a rich color in the diamond. The concentration of impurities in a diamond is determined using complex spectroscopic research methods (IR Spectroscopy, Optical Spectroscopy, Photoluminescence, etc.), which we will discuss in more detail in the following articles.
The picture below shows the color scheme of lab-grown diamonds, starting with colorless stones. The approximate concentration of impurity centers (in ppm), determined by instrumental methods, is compared with the color of the diamond. You can see the direct correlation between the color of a diamond with the concentration of nitrogen and boron, as well as, the required nitrogen concentrations needed to change the color of the diamond.
Thus, the color of a lab-grown diamond is one of its most important characteristics, the nature of which lies in the defect-impurity composition of the diamond crystal lattice. The color saturation of such a stone depends on the concentration of impurities (mainly nitrogen and boron). The entire spectrum of colored diamonds is available for reproduction in the laboratory (CVD and HPHT methods), which makes them relatively inexpensive and in demand in the market, compared to the naturally-occuring, pure diamonds, which are rare and expensive.
Author: LGDeal Gemology Department