Your Gateway for Buying & Selling Diamonds

You’ve read all the basics and you’re now ready to start looking for a diamond. What you’ll find once you start looking is that if you just input the 4Cs into a diamond search, you’ll end up with many choices that all look very similar. It can quickly get confusing not to mention being time consuming to go through each one looking to find the perfect proportions.

If you don’t remember the diamond proportions then you should revisit the diamond cut tutorial. In that tutorial, we talk about the Tolkowsky Ideal-cut (TIC) set of proportions. It turns out that there is actually a range of proportions that produces the top light performance characteristics that Tolkowsky identified.

Here are my recommended proportions for a diamond with balanced light performance:

Table: 53-57%
Total Depth: 60 – 61.5% (Max 62.5% for a 4% girdle)
Crown Angle (CA): 34 – 35 degrees
Pavilion Angle:
– For CA = 34, pavilion angle = 41 degrees
– For CA = 34.5, pavilion angle = 40.6 – 40.8 degrees
– For CA = 35, pavilion angle = 40.6 degrees
Lower Girdle: 75-80%
Star Length: 50-55%
Girdle: thin to medium preferred

Table Percentage

In most cases, picking a diamond with a 53-57% table alone reduced the number of diamonds to roughly the top 50% of GIA excellent stones available. If you find that you don’t have many choices, you can try to include 58% but it is not recommended to go any larger.

Total Depth (or simply Depth %)

Searching by crown and pavilion angles isn't common, there is a trick to pick diamonds that fall within this range. It is recommended to use the total depth, a common filtering value.

(Total depth = crown height + girdle + pavilion depth)

The crown angle and table size together determines the crown height and the pavilion angle directly corresponds to the pavilion depth.

Within our specified table percentage and crown/pavilion angles, the ideal crown height varies between 14-16% and the ideal pavilion depth between 42.5-43.5%.

In general, once you fix the size of the table, a steeper crown should be matched with a shallower pavilion and vice versa. Together with a thin to medium girdle, a simple calculation results in a range of total depth to be:

(min crown + max pavilion + min girdle) = (min total depth)
14 + 43.5 + 2.5 = 60%

(max crown + min pavilion + max girdle) = (max total depth)
16 + 42.5 + 3 = 61.5%.

Selecting a diamond with a total depth between 60 – 61.5% narrowed the number of diamonds to fewer than 6% of all GIA excellent stones from our tests. If you find you have too few choices, you can extend your criteria to include medium or slightly thick girdles if all other proportions are within our specifications, do this by increasing the total depth specification in increments of 0.5%. Just be aware that if the carat weight of the diamond is going into the girdle then its not going into making the diamond look bigger.

Diameter

The next trick is to pick the diamond’s diameter. Although you cannot filter by diameter, when you mouse-over the ‘view’ button, you will be able to see a short summary of the diamond in question and the measurements can then be seen quickly as you browse. Don’t be too strict with the diameter. If you pick a diamond that is below standard, check to see if its going into the girdle, although at the highest cost to spread, you want the weight to be going into a slightly thicker girdle if you had to.

Length-to-Width Ratio

The length-to-width ratio is not something that is commonly referred to when searching for round diamonds. You will see later that being round is very important to diamond optics. On GIA certificates, two measurements of the diameter are given, one is for the maximum and the other is the minimum. It is important to note that there is a 0.02mm margin of error in the measurement.

Choose a L/W ratio of 1.00. If you feel like you have too many choices then this is a good way to further narrow down your choices. Usually, adding this criterion reduced the number of diamonds to less than 3% of all diamonds. Having reduced the list of diamonds, you can pick diamonds that fit into your budget and size criteria followed by color and clarity to narrow down your top diamond picks.

Conclusion

The main takeaway from this tutorial is that filtering a large diamond database by cut is the most efficient way of finding top performing diamonds. A good way to filter diamonds is to use the advanced filter on most sites to look for diamonds with a 53-57% table, 60-61.5% total depth, and L/W ratio of 1.00. Make sure you check a diamond size chart to make sure you’re getting a diamond that is the right size for its weight.

Here is the round diamond proportions checklist again:

But don’t stop here because getting a diamond with these proportions can only guarantee light performance to some extent, you still need to learn about optical symmetry before you can search for a diamond effectively and have the confidence to go out and make that important purchase.

If you follow this search criteria and end up with very few or even no diamonds that meet all your specs, it’s easy to conclude that this criteria is too strict. You’re also unlikely to find a diamond that meets these specs at a your local jewelry store. If you get anything outside this range then you are essentially sacrificing cut for your other preferences. For H&A diamonds, you can be a bit more flexible with the CA/PA combinations (i.e. a 35/40.8).