Fancy color diamonds and fluorescence: beyond the Z grade

Lights out

Two diamonds sit side by side in daylight. Both are D, Flawless, Excellent. In daylight, nobody can tell them apart.

Now switch the room to ultraviolet light. One goes dark. The other lights up an intense blue, as if someone switched it on.

Same color grade. Same clarity grade. That glow is fluorescence, and it's one of the two topics of this page. The other topic is a diamond that has too much color for the D-to-Z scale.

Past Z: a fancy color

The D-to-Z scale ends with a master stone at Z. A yellow or brown diamond with more color than that leaves the scale completely. It is then treated as a colored diamond. When its color is strong enough, it gets a fancy grade.

A fancy color has three parts:

• its hue (yellow, orange, blue…)
• its tone (how light or dark)
• its saturation (how strong).

One intensity word sums them up. Colored diamonds aren't only yellow and brown. They also come in red, pink, blue and green, among others.

Here's the twist that surprises people. On the D-to-Z scale, less color is rarer. Past Z, it turns around: a stronger, purer fancy color is the prize.

Our simulator gives the fancy grades on 28 swatches, from Red through the yellows, greens, blues and purples to Purplish Red, plus Brown. They come from GIA's continuous hue circle. You can set each swatch to five intensities:

• Fancy Light: the palest fancy grade, a soft but definite color.
• Fancy: an even color of medium strength.
• Fancy Intense: strong color.
• Fancy Vivid: the strongest, purest color.
• Fancy Deep: strong color, darker in tone.

Two hues follow a special rule, the same as in GIA grading: red and purplish red get no intensity word at all. They are always Fancy Red or Fancy Purplish Red. And in our simulator, brown has only three intensities: Fancy Light, Fancy and Fancy Deep.

Fluorescence: the hidden glow

Some diamonds give off visible light while ultraviolet light shines on them. They stop at the moment the UV goes off. That's fluorescence. A report gives its strength under long-wave UV, from None through Faint, Medium and Strong to Very strong. It also gives its color.

GIA found this across the diamonds it grades:

• About 25 to 35% show some fluorescence.
• Only about 10% of those are Medium or stronger, the strengths that may change the look.
• In more than 95% of fluorescent diamonds, the glow is blue. Yellow, white and other colors are rare.
• In most diamonds, you won't see it at all in everyday light. In GIA's observations, people often liked the look of stones with medium to strong blue better.
• Fewer than 0.2% of fluorescent diamonds look hazy or oily from extremely strong fluorescence.

Fluorescence doesn't make a diamond weaker, and it isn't part of the color or clarity grade. The report lists it as an extra characteristic that helps to identify the stone.

Try it yourself

1. Open 3D Gemstone Simulator with the T73 in diamond. Set the lighting to UV lamp.
2. In the report, under Fluorescence, step from None to Very strong. Watch the glow build.
3. Leave it at Very strong and switch to Photo studio, then Sunny day. What changed?
4. Go back to Photo studio. Under Color grade, switch the scale from D to Z to Fancy color. Pick Yellow and step through the intensities. Then try Blue and Purplish Red.
5. The challenge: set a Z. Then find the weakest fancy yellow intensity that looks clearly more yellow than the Z.

What the simulator shows

The glow. The UV lamp is a dark room with only long-wave UV light. Under it, the non-fluorescent stone stays dark. The fluorescent stone lights up blue, and it gets brighter at each step. The simulator adds the glow along each light path inside the stone. So the deeper the light travels, the more it glows.

In daylight, nothing. Switch to any daylight scene, and the two stones look the same. That agrees with what GIA found for most diamonds. One simplification: real daylight has some long-wave UV in it. So a real fluorescent diamond does react a little outdoors. The simulator shows the glow under the UV lamp only.

The fancy colors. The intensity words go up from a soft Fancy Light to a saturated Fancy Vivid. Fancy Deep makes the color darker, not stronger. Red and Purplish Red give no intensity choice at all, and Brown gives only three. A fancy yellow is a different animal from a Z: its grade is about how much color it has, not how little.

Key points

• A yellow or brown diamond with more color than the Z master stone leaves the D-to-Z scale. With enough color, it gets a fancy grade.
• A fancy color has a name made of a hue and an intensity word. Stronger, purer color is rarer and more valued.
• Fluorescence is visible light that a diamond gives off under UV. A report gives its strength under long-wave UV and its color.
• About a quarter to a third of diamonds fluoresce, almost always blue. For most of them, it makes no difference to the look that you can see.
• Fluorescence isn't part of the color or clarity grade, and it doesn't make the stone weaker.

Go further: try the same Very strong stone in every lighting scene of the 3D Gemstone Simulator. Find the scene where you think the glow matters most in real life.