Synthetic diamonds, also known as lab-grown diamonds (LGD), only became a commercial reality in the gem market during the 21st century, although industrial-quality diamonds have been manufactured during the last fifty years.
There is generally no difference visible to the eye, or in the structure and physical properties of synthetic diamonds compared with natural diamonds. It is the manner in which they were formed that is different.
Two methods exist for producing lab-grown diamonds, one uses extremes of high pressure and high temperature (HPHT), almost replicating conditions where diamonds are formed deep within the Earth, and the other – chemical vapour deposition (CVD). Both methods lend themselves to growing diamonds of sizes up to and beyond 10 carats, though only HPHT is suitable for small diamonds, referred to as ‘melee’. By controlling impurities in the growth mix, different colours can be produced, but post growth treatment can also produce different colours. Irradiation can produce green and depending on the nitrogen content subsequent heating (annealing) can convert the colour to pink or red.
Natural diamonds can also be subjected to irradiation and or heat treatments if the nitrogen profile is appropriate. Diamonds with low nitrogen levels in a particular aggregation state can be converted to pink, while brown diamonds with negligible nitrogen can be rendered colourless from HPHT treatment. A natural diamond can also have its colour appearance changed by applying a thin coating on the underside of the stone. Such treatment can be applied to enhance the colour of a diamond, which can make it challenging to detect as all the signatures of the natural colour will be present.