Cut has two meanings in the diamond and jewellery industry.
The first meaning relates to the shape and style of a polished diamond, such as Round brilliant and Emerald cut. See diamond shapes.
The second meaning (in reference to the 4C's) is used to describe the relative quality of the proportions and finish of a polished diamond which is what will be explained briefly in the following paragraphs. In the diamond trade this is normally referred to as 'Make'. Finish refers to the quality of a diamonds polish, the condition of its girdle and the precision of the faceting.
The cut or make of a diamond is the most important of the 4C's. This is a very complex subject and is an on-going study by the major diamond grading laboratories. Make is the only one of the 4C's that man, the diamond cutter, can control. Polished diamonds should be considered optical instruments and ideally all light entering a diamond should be internally reflected back to the observer.
The diamond cutter should analyse a rough diamond to determine how to extract the most beauty out of that rough diamond. In a commercial environment they also cut a diamond to retain the most weight and therefore perceived increased value, to the uninitiated. This retained weight will be to the detriment of a diamonds face-up appearance and will be displayed as either very lumpy (small diameter) or very spread (wide diameter). This ties in with the minimum diameters that you should demand when purchasing round brilliants. See carat weight.
Diamonds cut with Excellent to Good proportions allow a maximum amount of light to be returned to the observer and thus have more brilliance and fire.
A diamond with a deep pavilion will appear dark in the centre of the diamond due to light leakage from the pavilion. Diamonds with a make grading of Fair to Poor will appear like this. Diamonds.co.nz does not list diamonds with Fair or Poor make.
A spread fisheye diamond with a shallow pavilion creates a white circle at the edge of the table when looking down on the diamond. This is actually the reflection of the girdle at the edge of the table. Diamonds with a make grading of Fair to Poor will appear like this. Diamonds.co.nz does not list diamonds with Fair or Poor make.
Something important to note is that any diamond other than rounds does not have a 'Cut' grade.