Spread is very important to you, but hardly ever talked about by technicians working away in labs. You want the diamond you are buying to look big. The diamond on the left of the photograph ‘sounds' big but looks smaller than the one on the right. Both measure 5.2 mm in diameter and could have been cut from the rough diamond in the centre. Note the lifeless one is 3.7 mm deep while the ideal cut is just 3.1 mm.
How do you know if the diamond you are considering has a good spread? If you divide the depth by the diameter and multiply by 100 you can calculate the depth percentage. This should be between 56% and 65%. If the table is small the depth will need to be larger, and vice a versa. The smaller the depth percentage, the larger the spread. Other factors that effect spread are the girdle thickness and crown and pavilion angles.
Holloway Cut Advisor rates a diamonds spread.