The genuiness and the value of larger diamonds (more than 30 points) is guaranteed by a certificate of authenticity. They customer pays extra for this guarantee of authenticity.
The best known certificates are issued by the Gemmological Institute of America (GIA) the International Gemmological Institute (IGI) and the Diamond High Council (HRD).
In the collection Real – Not real you can test your knowledge. Do you see the difference between a real diamond and one that is not? Some materials look a lot like diamond but are something else entirely.