The 6.61 ct diamond above shows the stone before and after HPHT annealing to remove its color. Left, prior to annealing, this diamond’s color was considered fancy yellow brown. Right, after annealing, this diamond’s color grade is "L" (faint yellow).

Diamond treatments are artificial methods used to improve a diamond’s color or clarity. In this article, you will learn about different types of diamond treatments, their stability, and how they impact a diamond’s appearance. Note that GIA grading reports list the presence of any known diamond treatments.

Why are diamonds treated?

Color Treatments

Clarity Treatments

Why are diamonds treated?

A relatively small percentage of gem-quality diamonds are treated to improve their appearance. Throughout history, diamonds were sometimes foil-backed in their settings to increase their brilliance. At the turn of the 20th century, scientists began experimenting with advanced diamond treatments, which were created for two primary reasons—to alter color or to improve clarity. These methods include coating, HPHT color treatment, laser drilling and fracture-filling. As science has advanced, treatments have become more sophisticated and harder to detect.

New coatings can turn diamonds virtually any color, as illustrated by these ~0.40 ct coated diamonds. Photo: C.D. Mengason/ GIA

What is diamond coating?

Coating is when a diamond is covered with an ultra-thin layer of chemicals or plastics to mask the diamond’s original color and increase the attractiveness of the diamond’s color. For example, blue coating on a yellowish diamond can neutralize its bodycolor and make the diamond appear more colorless. The ultra-thin coating is usually only applied to the pavilion or girdle region of a diamond and can be difficult to detect. It will scratch and wear off over time, however, and can be damaged by heat. Because this treatment is not permanent, GIA will not issue a report for a coated diamond.

The top row shows coated diamonds at room temperature. The following rows show what happens when coated diamonds are exposed to increasingly high temperatures. Photo: Jessica Arditi/ GIA

What is irradiation?

Exposing diamonds to radiation can change their color. Natural green diamonds are irradiated underground by nature. Now, humans can use high-energy electron irradiation in a particle accelerator or, rarely, neutron irradiation in a reactor to produce green to greenish blue diamonds. Some early methods of irradiation caused diamonds to have trace amounts of radioactivity but this no longer applies to diamonds irradiated in recent years. Sometimes the irradiation process is followed by annealing to increase color stability. Heating irradiated diamonds to above 1000o C will change most blue-to-green colors to brownish or orangy yellow, greenish yellow to yellow, or pink to red.

When this technology first developed a century ago, the colors created by irradiation were confined to the surface of the diamonds and could be lost if the diamond was repolished. Nowadays, technology has evolved so that the color reaches in more deeply. Nonetheless, irradiated diamonds are sensitive to heat; jewelry repair procedures, recutting, and repolishing can change their colors. Because the treatment is stable to regular wear, however, GIA will issue reports for irradiated diamonds.

These diamonds were irradiated and annealed to achieve their saturated yellow color. Photo: Robert Weldon/GIA

What is annealing?

Annealing is a controlled heating and cooling process that is often used after irradiation to further adjust a diamond’s color. Unlike HPHT, the temperatures reached are somewhat lower, and high pressure is not required. It is typically used to change diamond color to brown, orange, yellow, pink, red, and purple. The resulting color due to treatment is often highly dependent on the starting material and what defects are present in the stone. Generally, annealed diamonds are stable to jewelry repairs.

Heat treatment under high temperatures and low pressures can create black diamonds by causing graphite to form in surface-reaching fractures. The process is so commonplace that black diamonds treated this way are more prevalent that natural color black diamonds. Annealing is considered a stable treatment, so GIA will grade annealed diamonds.

Annealing turned this 0.085 carat, highly-fractured “milky white” diamond into a black diamond through graphitization of its surface-reaching fractures. Photo: Elizabeth Schrader/ GIA

What is HPHT color treatment?

HPHT is a method of annealing at high temperature and high pressure. The HPHT reactor can be used both to grow laboratory-grown diamonds and to change the color of natural or CVD laboratory-grown diamonds, making them colorless, pink, blue, green, yellowish green, or yellow. HPHT annealing is also often used to lighten the color of some brownish diamonds. This form of diamond treatment is stable, permanent, and difficult to detect except by a well-equipped grading laboratory.

The 6.61 ct diamond above shows the stone before and after HPHT annealing to remove its color. Left, prior to annealing, this diamond’s color was considered fancy yellow brown. Right, after annealing, this diamond’s color grade is "L" (faint yellow).

The 6.61 ct diamond above shows the stone before and after HPHT annealing to remove its color. Left, prior to annealing, this diamond’s color was considered fancy yellow brown. Right, after annealing, this diamond’s color grade is “L” (faint yellow). Photo: Robert Weldon/ GIA

These type II-A diamonds originally had color grades of J-M but their color grades improved to E-F color after HPHT treatment. Photo: Jian Xin (Jae) Liao/ GIA

This image shows natural diamonds that have undergone HPHT treatment to enhance their color. Photo: Robert Weldon/GIA. Courtesy: Iljin Diamond Co

What is laser drilling?

Laser drilling is a permanent treatment that enhances a diamond’s clarity by removing small, dark inclusions. The laser bores a tiny hole into the diamond’s interior to create a channel through which a bleaching agent can be introduced to lighten the inclusion’s appearance. The laser drill hole is considered a clarity characteristic. It typically goes straight into the diamond’s table or pavilion so that viewers may only see a tiny hole instead of a long, sideways channel.

Note that laser-drilled diamonds can be slightly more fragile than non-laser drilled diamonds, especially if the diamond is small in comparison to the drill tunnel. Dirt can sometimes become trapped in the tunnel, conversely making the diamond less attractive. Sometimes, people will fill the tunnel with a glassy resin to make it less visible. GIA grading reports will always mark laser drill holes in a diamond’s inclusion plot, but GIA does not grade diamonds that have been filled with a foreign substance. 

A newer method of removing dark inclusions is internal laser drilling. This method is used to treat diamonds with dark inclusions close to the surface. Instead of having a straight channel that reaches the inclusions, this method opens or expands a “feather” or cleavage to the surface of the diamond. This cleavage can resemble natural feathers within a diamond and be hard to detect as a treatment.

Seen here is a laser drill hole at 63x magnification. The round shape at the end shows where a dark inclusion was removed. Photo: Vincent Cracco/ GIA

Internal laser drilling can create a series of small step-like feathers that connect an inclusion to the surface of the diamond, often leaving behind a white area with a sugary texture in the center of the feathers. Magnified 37x. Photo: Vincent J. Cracco/ GIA

This is a sample GIA Diamond Grading Report for an internal laser-drilled diamond. Internal laser drilling is listed in the comments section.

What is fracture filling?

Fracture filling hides “feathers” or white fractures in a diamond. These white fractures are unattractive and can make a diamond appear cloudy if they are large or numerous enough. During treatment, a surface-reaching fracture is injected with a molten lead glass-like substance that fills the diamond and makes the fracture less visible. This treatment can help clients who accidentally crack their diamond.

The treatment can last for years with proper care, but it is not a permanent or stable treatment. The filling may be damaged or removed during repair or routine cleaning such as cleanings with steam, acid or ultrasonics. If the filler melts and leaks out, there is a possibility that it can be retreated. If the filler turns dark, it cannot be made colorless again. Fracture filling can make a diamond appear to have higher clarity, but it also slightly lowers a diamond’s color. Good fracture filling is very subtle, so examination by a skilled diamond grader is necessary to detect its presence in a stone. GIA does not grade diamonds that have been fracture-filled, because it is not a stable or permanent treatment.

Fracture-filled diamonds can display a rainbow flash effect where they were filled. Photo: Curtis Berry/GIA

Can treated diamonds receive GIA reports?

GIA only issues reports for treated diamonds if the treatment they received, such as laser drilling or HPHT color enhancement, is stable or permanent. GIA discloses the presence of treatments on the report, and as a further precaution, laser-inscribes the girdles of diamonds it identifies as color-treated. All GIA reports designate a diamond’s color as Natural Color or Treated Color. If you wish to know the type of treatment, you can order a separate Diamond Color Treatment Letter. GIA does not issue grading reports for diamonds that have received nonpermanent or unstable treatments, such as coating or fracture filling.

Seen here is the 104.53 carat Deepdene diamond, the largest known irradiated diamond in the world. Photo: Fabian Matzerath/picture alliance via Getty Images

Should you buy treated diamonds?

Treated diamonds can be beautiful and come with a lower price tag than untreated diamonds of similar color and clarity. There is nothing inherently wrong with treated diamonds so long as their identities are disclosed. Disclosure lets consumers know the identity and quality of what they are purchasing and helps them take better care of their diamonds, because while some treatments are stable, others are not. Color coated diamonds, laser-drilled and fracture-filled diamonds will require extra care.

While wholesalers and retailers are required to disclose the presence of treatments, the best assurance is to purchase a diamond with a GIA report.