- Facet - The plane surfaces of a diamond created by diamond polishing.
Round brilliant cut diamonds have 57 or 58 facets (with a culet), for example.
- Faceted Girdle - a girdle produced with approximately 32 small flat planes.
- Fancy Color - the color of the diamond, other than white (D to Z on the color grading scale).
Fancy colors include fancy yellow, fancy red, fancy blue, fancy green and fancy pink, among others. Fancy color diamonds are typically worth more than those diamonds in the D to Z color range, with blues, greens and reds worth more than any others.
- Fancy Shape - fancy shaped diamonds refer to all diamonds that are not round.
These include, but not limited to emerald cuts, cushions, radiants, Asscher cuts, ovals, marquises, pears and princess cuts.
- Feather - an internal inclusion that appears like a small white feather.
A feather affects the clarity grade and, thus, the diamond's value.
- Finish - the final detailing or finish of a diamond is comprised of polish and symmetry.
Polish is the overall condition of the diamond's surface. Symmetry is the exactness of the shape and placement of the facets.
- Fire - it is evident when white light is dispersed into its spectral colors.
Diamonds with larger tables tend to show less fire. However, the right combination of lower girdle facet length, crown angle and pavilion angle can still result in a fairly high level of fire.
- Fisheye - the dark reflection of the girdle within a shallow diamond.
Poorly cut diamonds (i.e. not deep enough) are worth less than a well proportioned diamond of a similar weight, color and clarity.
- Flaw - an inclusion, such as a blemish, fracture or cavity.
- Flawless - a diamond with no inclusions.
- Fluorescence - The light emitted by approximately diamonds when exposed to ultraviolet light.
If the fluorescence is strong, it can make the diamond look cloudy and thus lowers the value. For diamonds with a color of J or lower (K, L, M, etc.), fluorescence is not seen as detrimental to the appearance or value.