Facet of a diamond is the cut out of diamond. There are many types of facet of a diamond.
Facet is the flat planes that form the outer side of the diamond. The better the cut the more beautiful facet is.
Below is a diagram that describes the different facets in a typical round brilliant cut diamond.
The quality of a diamond might depend upon the cut of facet in which it reflects the brilliant, fire, sparkle and the luster of a diamond.