HPHT and CVD diamond growth processes
High Pressure High Temperature (HPHT) Diamonds
In the HPHT process, small diamonds are used as seeds. Graphite is dissolved in a molten metal at high pressure, and slowly precipitates as new diamonds.
Interestingly, the HPHT process can also be used to improve the colour of natural diamonds, changing them from brown to yellow-orange-green shades. This is a controversial process as it can be very difficult to identify natural diamonds that have had their colour improved by the HPHT process, even in the lab. Many of the labs who perform this process say that they are only restoring the natural colour of the diamond before it was discoloured in the volcanic process that carried it to the surface of the earth.
Natural diamonds treated by HPHT, if identified as such, should be significantly cheaper than the equivalent untreated diamond. Natural HPHT diamonds are required by law to be identified when sold or graded.
Chemical Vapour Deposition (CVD) Diamonds
Modern lab created diamonds are primarily produced by Chemical Vapor Deposition (CVD) when a very hot gas containing carbon is deposited into a chamber at high temperature and pressure. The carbon atoms are deposited onto a diamond ‘substrate’ which can be a naturally ocurring diamond, or one created by the HPHT method. The substrates or seed crystals grow new diamonds at up to half a millimetre per day. The equipment and method for making CVD diamonds is relatively simple compared to the HPHT process.
CVD diamonds can be produced with good clarity although they can also have brownish tints or faults. The clarity can be improved by using a HPHT process after the CVD diamond has been produced.
CVD diamonds can be difficult to distinguish from natural diamonds with the naked eye, or even standard gemmological instruments. Various lab tests can be used to identify them easily though, including Cathode Luminescense which identifies the growth pattern of the crystal. Some CVD diamonds may fluoresce under UV light, a feature rarely seen in natural diamonds.
There is a great article here about how CVD diamonds are made – I definitely recommend reading this if you are interested in the topic. This article is one of the things that got me really interested in how diamonds are made, and how they can be such a good alternative to the real thing!