Hot off the presses

Treating colored gemstones is nothing new. For centuries, colored stones have been routinely heated to produce more vibrant blues, eliminate silkiness in stones, and create an impression of higher clarity. In fact, it’s best to assume that the vast majority of gemstones have undergone such treatment, though disclosure is still crucial.

Today, however, the trade is taking the process of cooking stones one step further and applying High Pressure-High Temperature (HPHT) treatment to sapphires. While some may already be familiar with this procedure from its role in improving diamond color and clarity — as well as growing synthetic diamonds — the purpose of using HPHT on sapphires is to help lower-grade stones that did not improve with normal heating attain the beautiful blue that consumers demand.

Entering the limelight

Small-scale HPHT processing for sapphires began in 2015, according to Eddie Cleveland, a researcher at GemResearch SwissLab (GRS). “More and more locations are acquiring hydraulic presses, including our own machine for processing [the stones and for continuing our] research.”

HPHT provides consumers with richly-hued sapphires in larger carat sizes than they might otherwise be able to afford. Stunning specimens over 20 carats have come on the scene.

However, HPHT sapphires may have been on the market for some time already without vendors or retailers even being aware of them. Buyers of smaller goods usually don’t ask for as many details about treatment, so there is less incentive to have the stones tested for it.

“Getting individual certification may cost more,” explains Zion Pavithra de Silva, a wholesaler at Mt. Zion Gems in Sri Lanka. As such, he says, sellers often leave the matter ambiguous and “just say their goods may or may not be heated.”

The exact percentage of HPHT sapphires traded globally is unknown, but estimates place it in the 3% to 5% range. Because these stones would have been unsalable without the treatment, manufacturers may offer them at lower prices, but there’s less of a discount as they get further down the chain.

What to look out for

The call to disclose this additional treatment is a vote for market transparency, but it also has practical implications for vendors, manufacturers and end users.

While there’s no conclusive data showing HPHT sapphire is more brittle than conventionally heated stones, there is growing caution on that front. GRS has run tests to determine the treatment’s effect on sapphires’ durability, and after sawing and hammering 14 of these stones, the lab found some were indeed less durable. Of course, since it’s impossible to know a stone’s history with certainty, it could have undergone multiple heat treatments prior to HPHT, and that may be a factor in its brittleness.

One dealer recently corroborated this theory, according to GRS founder Dr. Adolf Peretti. The dealer told him about a large sapphire that was recut to eliminate a girdle chip, after which two new chips appeared on the girdle. Sending it back again for a remedial recutting resulted in even further chipping, Peretti relates.

Another notable outcome of the treatment is that some HPHT sapphires display color zoning — an uneven distribution of color — close to the stone’s surface, says the GRS founder. This is important because most of these goods are native cuts — gems cut locally in their countries of origin — that require repolishing and shaping to conform to Western standards. But recutting these stones may mean cutting away the concentrated outer layers of color, a move that could negatively impact their beauty.

The importance of disclosure

Peretti proactively encourages the global trade to take a unified stand on requiring disclosure and transparency for HPHT sapphires, and to standardize the terminology for them. So far, an industry-wide agreement has yet to materialize.

HPHT stones certainly have their place in today’s market, but as Peretti points out, they must be identified so all parties can buy knowledgeably. When retailers know what they are selling, they can inform their customers of the advantage this process offers, delivering a vibrant sapphire at prices they can afford.

HPHT at a glance

Before the advent of HPHT sapphire processing, corundum — the mineral family to which sapphire belongs — was conventionally heated at temperatures between 1,500 and 1800 degrees Celsius. Yet some stones still did not improve enough to become salable.

Then, manufacturers parlayed the concept of HPHT, which had proven successful in improving the color and clarity of low-end diamonds, onto these sapphires. The process involves placing the stones in highly pressurized containments at approximately 1 kilobar of applied pressure, or 1,000 times atmospheric pressure, for 15 to 30 minutes. The results are remarkable.

On careful inspection, one can distinguish between regular heated sapphires and ones that have undergone HPHT. Microscopic examination with a Fournier transform infrared (FTIR) microscope reveals color zoning in the form of a halo rim, as well as three-dimensional fractures around crystal inclusions, and brownish staining in the fractures.

Many labs have not invested in this critical technology, although some labs have standard FTIR instruments to identify oils, resins, heat treatments or the type of stone. An FTIR microscope is necessary in research because the spectroscopic data it produces makes it possible to distinguish the patterns of the stone’s core from those of its rim. If it’s been treated with both heat and pressure, the HPHT signature will show up in the rim, while the core will show a conventional heat signature.

In iron-rich stones, HPHT produces rims that are blue and a few millimeters deep on some gems, but the color can penetrate to the centers in certain cases. Stones in the medium color ranges in particular can appear to be equally zoned. Since HPHT sapphires are mostly native cuts that will be recut to Western standards, they risk losing some of the intense blues that characterize the external rim.

So far, the processing of HPHT sapphires has taken place in South Korea, but other countries will likely begin performing this treatment as well in the future.

Image: Shutterstock

Article from the Rapaport Magazine - May 2019. To subscribe click here.