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This lesson tackles one of the most misunderstood and controversial topics in gemology. In the trade, fluorescence can be a “secret weapon” for value, or a “deal-breaker” for beauty.

Diamond Fluorescence: The Hidden Glow

The Diamond that Changes in the Sun

Imagine showing a client a beautiful diamond indoors. It looks perfectly white. You step outside into the South African sun, and suddenly, the diamond has a soft, ghostly blue glow.

Is it a superpower? Is it a defect? In the world of professional grading, Fluorescence is the “X-factor” that can either save you thousands of Rands or make a diamond unsellable.

1. What is Fluorescence?

Fluorescence is a diamond’s reaction to Ultraviolet (UV) light. About 25% to 35% of natural diamonds contain sub-microscopic particles (usually nitrogen) that emit a visible glow when exposed to UV rays (which are present in sunlight and blacklights).

The Most Common Color: Blue (95% of the time).
Other Colors: Yellow, Green, or Red (extremely rare).
The Grading Scale: GIA grades fluorescence as: None, Faint, Medium, Strong, and Very Strong.

2. The “Blue Trick”: Making Yellow Diamonds White

This is where the pro makes their money. Blue and Yellow are complementary colors—they cancel each other out.

The Value Play: If you have a diamond with a lower color grade (like J, K, or L), it has a slight yellow tint. If that stone has Strong Blue Fluorescence, the blue glow masks the yellow tint in daylight.
The Result: The diamond looks 1 or 2 color grades whiter than it actually is. You buy a “J” at a discount, but it looks like an “H.”

3. The “Oily” Trap: When Fluorescence Fails

Fluorescence isn’t always a friend. In high-color stones (D, E, and F), there is no yellow tint to mask.

The “Over-Blue” Effect: In rare cases, “Strong” or “Very Strong” fluorescence can interfere with the diamond’s transparency. It creates a “milky,” “oily,” or “hazy” appearance.
The Price Hit: Because of the risk of cloudiness, the trade applies a discount to top-tier colorless diamonds (D-F) that have strong fluorescence, even if they aren’t actually milky.

4. The Professional Field Test

As a grader, you need to know if the fluorescence is helping or hurting the stone.

Check the Certificate: See if it’s “Strong” or “Very Strong.”
The Sunlight Test: Take the diamond into direct South African sunlight. Does it look crisp and clear, or does it look like someone smeared a tiny bit of oil on it?
The Transparency Check: Look at the facet edges. If they look “blurred” under UV light, the fluorescence is too high.

The Quick Win: The “Blue Discount” Strategy

Here is the secret to getting the best deal in the market:

Find an “I” or “J” color diamond with “Medium” to “Strong” Blue Fluorescence that has been verified as “NOT Milky.”

You will get a stone that looks like a high-end “G” color to the naked eye, but you’ll pay the lower-tier price. This is how professional dealers source stones for their private clients to maximize the “sparkle-per-rand.”

Summary for Students:

Fluorescence is not a grading of quality; it is a characteristic.
In Lower Colors (I-M): Fluorescence is a Benefit (makes them look whiter).
In Higher Colors (D-F): Fluorescence is a Risk (can make them look hazy and lowers resale value).