Yellow diamonds are the most common of the fancy colored diamonds from among the high end jewelry class of fancy color diamonds which include Yellow diamonds, Pink diamonds, Blue diamonds, Green diamonds and Orange diamonds. We choose to exclude Brown diamonds from that category even though there are many 'value jewelry' manufacturers currently marketing Brown diamonds as 'Chocolate' diamonds. That being said, yellow diamonds are incredibly rare when compared to colorless diamonds of any grade.
The statistic most commonly presented is that for every 10,000 carats of diamonds mined, only 1.00 carat is a fancy color diamond. Taking that one step forward, for every 1,000 carats of fancy colors, roughly 100 carats will be yellow diamonds. Congac or Brown diamonds account for the vast majority of fancy colors mined, probably somewhere close to 85% of that 1,000 carat total.
With Yellow diamonds accounting for roughly 100 carats, the remaining 50 carats of fancy colored diamonds are split unevenly between the incredibly rare red, pink, blue, green and orange diamond colors. So in terms of diamond to diamond comparison, yellow diamonds are quite rare and in our estimation, vastly undervalued.
Like all fancy colored diamonds, yellow diamonds are graded on a 9 color intensity/tonality scale. However, there are only 4 grades within this scale that are considered desirable colors for yellow diamond jewelry. The color grade can be thought of on a linear scale where you have low color saturation on the left side and as you move toward the right side the saturation increases and the diamond appears more yellow.
The remaining 'less desirable' grades for yellow diamonds are:
Faint - Very Light - Light - Deep - Dark
Since there is no single color in the spectrum that perfectly represents a Fancy Yellow you can see how there is some overlap between the grades. For example, the very best Fancy Yellow diamond is virtually indistinguishable from the lowest example of an Intense Yellow, the same way the lowest Fancy Yellow would be identical to the best Fancy Light Yellow. This is where experience and expertise come into play. Getting the basics from the GIA report is just the first step in the process of evaluating a yellow diamond.