Diamond treatments are processes that improve a diamond's apparent clarity or change its color after it leaves the mine. The main ones are laser drilling, fracture filling, HPHT processing, irradiation with annealing, and surface coatings. Treated diamonds are real diamonds, but they sell for less than comparable untreated stones, some need special care, and sellers should always disclose them.
Why diamonds are treated
Most mined diamonds have inclusions or tints that lower their price. Treatments make a lower-grade stone look like a higher-grade one, or turn a stone of unpopular color into one buyers want. Some are permanent and some are not. The broader picture for all gems is covered on the gemstone treatments page.
Clarity treatments
Laser drilling
Laser drilling targets dark inclusions, such as black crystals, that stand out against the white body of a diamond. A laser burns a very narrow channel from the surface to the inclusion. The diamond is then boiled in acid, which travels down the channel and bleaches or dissolves the inclusion so it becomes much less visible.
The technique has been used since the 1960s and 1970s. It is permanent, though the drill hole stays as a fine white tunnel, and the hole can let dirt in.
A later form, sometimes called internal laser drilling or KM treatment (from the Hebrew kiduah meyuhad, "special drill"), uses a laser to extend or create small feathers from an inclusion to the surface instead of a straight tunnel. Acid then reaches the inclusion through those feathers. The result can look like natural cleavage cracks.
GIA and other labs grade laser-drilled diamonds and note the drilling in the comments, and the drill holes appear on the clarity plot. The diamond clarity guide covers how inclusions are graded.
Fracture filling
Fracture filling targets feathers and cracks that reach the surface. A high-refractive-index glass, often lead-based, is forced into the cracks under heat and vacuum. Because the glass has a refractive index close to diamond's, light passes through the filled crack instead of reflecting off it, so the feather becomes much harder to see. Filling became commercial in the 1980s.
Fracture filling is not permanent. The glass can be damaged by:
- Heat from a jeweler's torch during repairs or sizing.
- Ultrasonic and steam cleaning.
- Acids used in some jewelry pickling solutions.
- Repolishing.
Damage can leave the filler cloudy, cracked or drained, so the feather reappears and the stone looks worse than before treatment. A jeweler working on a filled diamond needs to know it is filled.
The usual sign of filling is the flash effect. When the stone is tilted under magnification, filled cracks show flashes of color, often orange-yellow or purplish pink in some positions and blue or green in others. Flattened gas bubbles in the filler and a crackled texture also give it away. GIA does not grade fracture-filled diamonds, though it identifies them.
Color treatments
HPHT processing
High pressure high temperature (HPHT) treatment subjects a diamond to conditions similar to those deep in the Earth, using the same kind of press used to grow lab-grown diamonds. The process became commercial for gem diamonds in the late 1990s.
The effect depends on the diamond type.
- Brown Type IIa diamonds can become colorless or near-colorless. Their brown color comes from lattice distortion, and HPHT repairs some of it.
- Some Type IIa stones can become pink, and some boron-bearing Type IIb stones blue.
- Brown Type Ia diamonds, which contain nitrogen, can turn yellow, greenish yellow or yellowish green.
HPHT color is permanent and stable under normal wear and jewelry repair. It is hard to detect without laboratory spectroscopy, and labs check for it routinely, especially in colorless Type IIa stones. A diamond that has been HPHT-treated should have that noted on its report, and major labs inscribe "HPHT processed" or a similar phrase on the girdle. The diamond chemistry and formation page explains diamond types.
Irradiation and annealing
Irradiation exposes a diamond to high-energy particles, such as electrons in a linear accelerator or neutrons in a reactor. The radiation displaces carbon atoms and creates defects that absorb light, usually turning the stone green, blue-green or black.
Heating (annealing) after irradiation lets those defects move and combine with nitrogen, which shifts the color. Depending on the starting diamond and the process, irradiation followed by annealing can produce yellow, orange, brown, pink, red or purple.
Irradiated color is stable in normal wear, but high heat from a torch can change it, so jewelers should be told. Labs detect it with spectroscopy and by looking at color zoning, such as color concentrated around the culet in a pattern that matches the cut shape (the "umbrella effect" seen in some cyclotron-treated stones). Treated stones in all of these colors are much more common than natural ones, as explained on the fancy colored diamonds page.
Irradiated diamonds sold through normal trade channels are not radioactive at levels of concern; electron treatment does not induce radioactivity, and neutron-treated stones are held until any residual activity decays.
Coatings
Thin surface coatings can make a diamond look whiter or add color. Old methods included painting the pavilion with a bluish ink to offset yellow tint. Modern methods apply very thin films, such as silica-based layers with metal or diamond-like carbon, to the pavilion or the whole stone.
Coatings are not permanent. They can scratch, wear or come off with cleaning, chemicals or repairs. Labs detect them with magnification, looking for scratches or uneven color on the surface, and with spectroscopy.
Combined treatments
Some diamonds receive more than one treatment, such as HPHT followed by irradiation and annealing to produce pink or red, or a filled stone that was also drilled. A lab report should list every treatment found.
Disclosure
In the United States, the FTC's Jewelry Guides say a seller should disclose a treatment if it is not permanent, if it creates special care requirements, or if it has a large effect on value. Fracture filling, coatings, irradiation and HPHT all fall under these rules in most cases. International trade bodies such as CIBJO publish similar standards. Grading reports from major labs state treatments in the comments, and the guide to diamond certificates explains where to find them.
Effect on value
Untreated diamonds of a given grade carry the highest prices. Treated stones sell at discounts that vary by treatment.
| Treatment | Permanent? | Special care | Typical value effect |
|---|---|---|---|
| Laser drilling | Yes | Little | Lower than untreated stones of the same grade |
| Fracture filling | No | Avoid heat, ultrasonic, steam, acids | Large discount; GIA will not grade |
| HPHT | Yes | Little | Below natural color equivalents |
| Irradiation and annealing | Yes in wear | Avoid high heat | Far below natural fancy colors |
| Coating | No | Avoid abrasion and chemicals | Large discount |
Treated diamonds can be good value for a buyer who knows what they are buying. A filled diamond can look much better than its untreated grade for a fraction of the price, as long as its owner and jeweler give it the right care. The how to tell if a diamond is real guide covers testing, and the jewelry cleaning page covers safe cleaning.
Frequently asked questions
Are treated diamonds real diamonds?
Yes. Treatments change apparent clarity or color, but the stone is still diamond. It must be disclosed and is worth less than an untreated stone of the same appearance.
How can you tell if a diamond is fracture filled?
Under magnification, filled cracks show colored flashes when the stone is tilted, often orange or pink in one position and blue or green in another.
Is HPHT treatment permanent?
Yes. HPHT color is stable in normal wear and repair, but it must be disclosed, and labs detect it with spectroscopy.
Can I clean a fracture-filled diamond in an ultrasonic cleaner?
No. Ultrasonic and steam cleaning can damage the filler. Use warm water, mild soap and a soft brush.