Cut is the single most important factor in the appearance of a diamond, and should never be underestimated when selecting a stone. Diamonds that have been poorly cut can appear dull and sometimes have dark spots on them, whilst well-cut stones can be enchanting, brilliant and fiery - exactly what we expect a diamond to be like.

There are three accepted sets of proportions that are considered ideal for cutting diamonds. These are known as:

• The Tolkowsky Cut (or American Standard Cut): This was first calculated in 1919 by Marcel Tolkowsky, and is believed to offer the best balance of brilliance and fire.
• The Eppler Cut (or European Cut)
• The Scandinavian Standard Cut (Sometimes known as the Scan D.N. Cut)

The above three types of cut all have a diamond table diameter of between 53% and 57.5%, a crown height of 14.4% - 16.2% and an overall height of 57.7% - 59.3%, with the exact specifications varying slightly between the three cuts.

Although the parameters used for these three cuts are considered ideal, not all diamonds can be cut to ideal specifications - in fact less than 1% of stones are used for cutting in this way. A key reason for this is that ideal cut stones can result in a greater amount of waste, and as a result are only performed by the very finest diamond cutters in the world. An example of this is that only three diamond cutters in the US currently perform the Tolkowsky cut.

It is possible to judge the overall quality of a cut of a diamond without making the above measurements.

The following are all signs of a poor cut:

• A white ‘donut’-like circle in the face-up view of the stone: This means the pavilion is too shallow and the girdle is being reflected out.
• Visible dark spots when viewed face-up: These mean the pavilion has been cut too deep.
• Thick girdles: These make the stone appear small for its weight and reduce the brilliance of the diamond.

Things you should look for in a diamond:

• Make sure that the culet is not chipped or broken.
• The girdle should not be too thick or too thin.
• The girdle should also be straight, not wavy from the sides.
• The facets of the crown should align with the facets of the pavilion, as any misalignment will display itself in the girdle.
• Stones that have been cut in a hurry may have small cracks that extend into the stone, giving it a ‘fuzzy’ appearance. However, this should not be confused natural frosty color of the girdle.
• The table should be parallel to the girdle, and the culet (when viewed through the table) should be centered and undamaged.