A synthetic, or laboratory-grown, diamond is made of crystallised carbon exactly like a natural one: same chemical composition, same crystal structure, same hardness. This is why the eye, and often even a ten-power loupe, is not enough to tell them apart.
They are produced by two main methods: HPHT, which reproduces the high pressures and temperatures found underground, and CVD, which grows the crystal from a gas. Each method leaves subtle traces in the structure and in the tiny internal imperfections.
The gemmologist uses instruments such as the spectrometer, ultraviolet light and devices designed to observe fluorescence and growth lines. The result is set out in a report that also states the origin: natural or laboratory-grown.
For buyers the advice is simple: ask for the diamond to come with a certificate from a recognised gemmological laboratory and, if in doubt, consult a qualified professional.