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SYNTHETIC DIAMONDS

Diamanti accresciuti in laboratorio

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SYNTHETIC DIAMONDS HPHT

SYNTHETIC DIAMONDS HPHT - WHAT IS A HIGH-PRESSURE HIGH-TEMPERATURE (HPHT) DIAMOND?

HPHT stands for high pressure, high temperature and is one of the primary methods used to grow diamonds in a lab. This diamond growth process subjects carbon to extreme temperatures and pressures and is meant to replicate the extreme heat and pressure conditions deep within the earth where natural diamonds form.

THE HPHT DIAMOND GROWTH PROCESS

Diamond seed is placed in a specifically designed press. The growth chamber is heated to 1300-1600 °C with pressures above 870,000 pounds per square inch. The molten metal dissolves the high purity carbon source.

Carbon atoms precipitate on a small diamond seed crystal, and a synthetic diamond begins to grow. The lab-grown crystal is then cut and polished by a diamond cutter.

In greater detail, HPHT diamond growth takes place in a small capsule inside an apparatus capable of generating very high pressures. Within the capsule, a carbon starting material, such as graphite, dissolves in a molten flux consisting of metals such as iron (Fe), nickel (Ni) or cobalt (Co), which lowers the temperature and pressure needed for diamond growth. The carbon material then migrates through the flux towards the cooler diamond seed and crystallizes on it to form a synthetic diamond crystal. Crystallization occurs over a period of several days to weeks to grow one or several crystals.

While natural diamond crystals tend to form as octahedrons, HPHT synthetic diamond crystals typically have cubic faces in addition to octahedral ones. Because the shapes of natural and HPHT synthetic diamond crystals are different, their internal growth patterns also differ dramatically. These growth patterns are among the most reliable ways to separate natural from synthetic diamond crystals.

The resulting faceted synthetic gems often exhibit distinctive color distribution, fluorescence zoning and graining patterns related to their cross-shaped growth-sector structure and also sometimes contain dark flux-metal inclusions. In some cases, the material exhibits phosphorescence, glowing after being exposed to ultraviolet light even when the light source has been turned off.

SYNTHETIC DIAMONDS - CVD

SYNTHETIC DIAMONDS CVD - WHAT IS A CHEMICAL VAPOR DEPOSITION (CVD) DIAMOND?

CVD stands for chemical vapor deposition and is another method used for making diamonds in a lab. Diamonds grow from a hydrocarbon gas mixture subjected to moderate pressures and temperatures in a vacuum chamber.

THE CVD DIAMOND GROWTH PROCESS

Diamond seed crystals are placed in a diamond growth chamber.

The chamber is filled with carbon-containing gas.

The chamber is heated to about 900-1200°C.

A microwave beam causes carbon to precipitate out of a plasma cloud and deposit onto a seed crystal.

Diamonds are removed every few days to have the top surface polished to remove any non-diamond carbon before being put back in to grow. Each batch of diamonds may require several stop/start cycles, and the entire growth process can take three or four weeks.

After the synthetic diamond crystals are removed, they are ready to be cut and polished into the final product.

In greater detail, CVD diamond growth takes place inside a vacuum chamber filled with a hydrogen and carbon-containing gas, such as methane. A source of energy — such as a microwave beam — breaks down the gas molecules, and the carbon atoms diffuse towards the colder, flat diamond seed plates. Crystallization occurs over a period of weeks, and several crystals grow at the same time. The exact number depends on the size of the chamber and the number of seed plates. The tabular crystals often develop a rough edge of black graphite that needs to be cut away. They also display a brown color that can be removed by heat treatment prior to faceting. Most CVD-grown colorless material on the market were probably once brown crystals that have been decolorized by HPHT annealing.

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