Editor’s Note
This article discusses HPHT (High Pressure High Temperature) treatment, a process used to enhance the color grade of natural diamonds. It is important to distinguish this treatment from HPHT synthesis, which creates lab-grown diamonds. The article originally conflated these distinct processes. The text has been amended for clarity.
HPHT technology involves treating natural diamonds under high temperature and high pressure to enhance their color grade. Generally, it can upgrade a diamond by 4 to 6 grades. However, not every diamond is suitable for this treatment; it must be of J color or better, and be of high clarity without inclusions.
In September 2016, the GIA laboratory in Hong Kong examined a 10.08-carat blue diamond synthesized using the High Pressure High Temperature (HPHT) method, which is the largest HPHT synthetic diamond on record.
The increase in the circulation of synthetic diamonds has drawn the attention of the Central Gem Laboratory (CGL) in Tokyo, Japan. Since the autumn of 2015, there have been more and more cases where small diamonds imported by wholesalers were mixed with synthetic diamonds. There is also the possibility of synthetic diamonds being circulated as commercial goods, which has a certain impact on the valuation of buying shops.
Until now, synthetic diamonds produced in China have been mainly used for industrial purposes, utilizing their hardness for cutting and grinding construction materials. However, due to the impact of China’s economic slowdown on the construction industry, the demand for synthetic diamonds has decreased. When market prices fell significantly, research and improvements in jewelry production led to more instances of synthetic diamonds being mixed with natural diamonds during circulation.
In November 2016, the Central Gem Laboratory began selling a kit called “CGL Diamond Kensa” for roughly screening colorless diamonds. The tool categorizes diamonds into Type I and Type II based on their ultraviolet transmission. If it’s Type I, it is likely a natural diamond. If it’s Type II, there is a higher possibility it is a synthetic diamond. This tool can make a distinction in about four seconds.
Synthetic diamonds for jewelry have not posed a significant problem so far due to technical difficulties in creating larger diamond patterns and the option to have second-hand gems appraised by institutions. However, Mr. Kitawaki pointed out, “Larger-pattern synthetic diamonds may circulate in the future.” He added, “There have already been successful productions of 10-carat synthetic diamonds in the world, and there is a possibility they could be maliciously sold to buying shops.”
The Gemological Institute of America (GIA) also published a 24-page article titled “Observations On HPHT-Grown Synthetic Diamonds: A Review” in the Autumn 2017 issue of G&G, detailing the characteristics and identification features of HPHT synthetic diamonds. The study used thousands of samples, encompassing all HPHT synthetic diamonds examined by GIA between 2007 and 2016!
Natural diamonds are almost non-magnetic, but HPHT synthetic diamonds, due to the use of metal catalysts, have a higher chance of containing residual metal, allowing detection based on metal magnetism.
Typically, diamonds below 0.05 carats are called “melee diamonds.” HPHT synthetic melee diamonds have only appeared in the jewelry market in the last two or three years, but their quantity is large enough to raise concerns. The HPHT synthetic melee diamond industry in Henan, China, has developed rapidly. In 2016, each factory could produce 1,000 carats of melee diamonds per day, with color grades ranging from D to N, cut weights from 0.005 to 0.03 carats, and varying clarity.
Based on recent data, GIA gemologists predict that the proportion of HPHT synthetic diamonds will continue to increase in the future. Among them, the number of HPHT synthetic diamonds above 4 carats and below 0.05 carats will continue to rise. Yellow series diamonds will still dominate but decrease in quantity, while blue and colorless diamonds will continue to increase.
HPHT diamonds can also have certificates issued by internationally authoritative diamond grading laboratories. GIA provides grading services for synthetic diamonds, issuing grading reports for loose stones over 0.15 carats and identification reports for synthetic diamonds of any size and color, clearly labeling them as “Synthetic Diamond” on the report.
Selling synthetic diamonds itself is not illegal, so it is impossible to prevent customers from selling synthetic diamonds as natural diamonds to buying shops. Furthermore, mixing natural and synthetic diamonds in retail sales could cause consumer panic. Once a mistaken purchase occurs, it could cause immeasurable losses to the store.