Although diamonds are held In the highest esteem, especially because of their colorlessness, it is the eccentric caprices of nature which in rare cases, also give a diamond imaginative colors, for example, red tints as a rare hue, and with decreasing rarity, green, blue, purple, brown and yellow. Especially with yellow tints there are diverse hues, and as many terms, such as champagne, canary, lemon, and gold-yellow.
Eccentric and emphatic natural fancy-colors in
diamonds nowadays fetch truly connoisseur prices at international
auctions which far exceed the price of colorless diamonds. The deeper
the color tint, the higher the price as an expression of its natural
rarity.
As with all color gemstones attempts were made as far back as the beginning of this century to change the natural color by artificial intervention.
The beginnings of diamond coloring by artificial irradiation go back to scientific experiments carried out in 1904 by the Englishman, William Crookes, with radioactive metal radium. To general surprise it was ascertained that stones which are colorless and tinted from light-yellowish to brownish changed, after a fairly lengthy exposure of several weeks, into a range of hues from radiant deep blue (like topaz) to blueish-green. color and radioactivity were stable.
However this method had no commercial impact, as the diamonds became radioactive from the radiation and anybody who wore these stones was in danger of radioactive burns.
Further developments in artificially induced color changes in diamonds resumed only after the Second World War. Cut diamonds were irradiated in the cyclotron with protons (positively charged particles) and deutrons (nucleus of the hydrogen isotope deutrium, consisting of a single proton and a neutron).
Under the influence of particles of varying ionizing intensity, penetration into the crystal lattice results in atoms being torn out of their original positions and deposited at the so-called interstitials, while at the same time they leave behind a vacancy in their original position. This artificially produced lattice disturbance (Provided it is kept within limits) causes a change in the selective absorption ratios in the stone which are the cause of the visually perceptible color change. However the color change thus produced achieves only a penetration depth of 0.2 mm to 0.3 mm in the cyclotron, so that a renewed cut or polishing of the stone leads to removal of the artificial layer of color. The radiation colors are green hues which, depending on the duration of irradiation in the reactor can develop from bright green through tourmaline-green to dark green.
With subsequent heating from around 500° to 800°C, these green hues change into gold-yellow brown and red-brown. In rare cases pink and red hues appear as exceptional chance products.
In the course of continuing development, irradiation was carried out nuclear reactors with neutrons (negatively charged particles) which lead to a complete penetration of a stone, so that even rough diamonds and not merely cut stones can be treated.
In a nuclear reactor light-green colors can be obtained with a short irradiation period, while with a longer irradiation period, blue-green to dark-green hues can be obtained which with subsequent heating to 500°C change to varied fancy colors depending on its structure. Thus
| Type I a | Yellow to amber colors |
| Type I b | Brownish red to brownish purple |
| Type II a | Intensive brown |
| Type II b | Intensive blue-green |
The diamond types depend on the inter-stratification of the various trace elements.
Type I a and Type I b have a small percentage of nitrogen. Nearly all natural diamonds belong to type I a. Nearly all forms of diamond synthesis crystallize in Type lb.
Type II a has no nitrogen whatsoever, is rare and has special physical properties. It is somewhat harder and tougher.
Type II b is very rare and contains boron: hence its semiconducting properties (+ aluminum = natural blue).
In the Van-der-Gratt generator diamonds are
bombarded with electrons. However, because of their relatively low
energy the depth of color is only around 0.2 mm to 0.5 mm. Green-blue,
pale blue and blue-green color hues at first and then blend into
the above-mentioned colors as a result of subsequent heating.
Artificially colored diamond must be designated in the
trade in such a way that the artificial color be clearly understood
from the designation, as or example, "artificially colored",
"bombarded", "irradiated" or "treated".
| Artificial color Changes in Diamonds and their Method of Identification | Distinguishing features |