The quality of a diamond is graded along different characteristics:
The internationally most commonly used scales for these characteristics were developed by the Gemological Institute of America (GIA). The GIA is the internationally most highly respected and reliable institute for assessing the quality of diamonds.
In the best case your diamond should therefore be certified by the GIA. Another trustworthy institute is for example the HRD (High Diamond Council) based in Belgium. With other institutes it can happen that diamonds are attributed to quality levels that they do not correspond to at all in order to achieve a higher price.
→ More information about the GIA certificate