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Ask a Diamond Pro: How Big of a Deal is Fluorescence?

By Mike Fried,

Occasionally, we’re contacted by readers who’ve found a diamond that looks great, but has one small problem that’s making them feel concerned.

For example, a reader recently contacted us and said that they were worried about a diamond with strong blue fluorescence. Our answer was that strong fluorescence often gives a diamond a hazy appearance, and should generally be avoided.

Here’s the reader’s full message:


You helped me out a few years ago and I’m returning to the hunt for a marquise cut, preferably below $5000. I came across the following: 


However, I’m concerned about the “strong blue fluorescence”. I would love your expert input. 


This is quite a common situation. Using the search features on James Allen and Blue Nile, it’s easy to find diamonds that have the optimal cut quality, clarity, color and carat weight, but not everyone knows what to do with the fluorescence filter. And that’s something you might feel concerned about. The quick answer is to avoid fluorescence on such a high color grade he chose – E color.

In this case, the reader is looking for a diamond in the $3,000 to $5,000 range but has worries about this stone’s fluorescence. 

Is Diamond Fluorescence That Big of a Deal?

So, what is fluorescence, and why is strong fluorescence a bad thing? Fluorescence refers to the glow that you might see when a diamond is viewed under ultraviolet light, such as natural sunlight or a blacklight. 

As the American Gem Society explains, “fluorescence occurs when a substance absorbs the energy from a source of electromagnetic radiation, usually ultraviolet (UV) light, and is then immediately released by the material, as visible light, in a continuous “in and out” flow of energy.”

Approximately 30 percent of diamonds show some fluorescence, meaning they glow if they’re exposed to UV light. The overwhelming majority of these diamonds glow blue, although some diamonds display different colors. 

Fluorescence in diamonds can be negative or positive. In some cases, it enhances a diamond’s perceived color and makes it look more white. In other cases, it can give the diamond a hazy look that’s definitely not ideal.

Let’s take the diamond the reader was considering and use it as an example of how a strong level of fluorescence can be a negative feature in a diamond.

The diamond the reader shared is a 1.02 carat, E color, VS2 clarity marquise diamond. It’s an eye-clean diamond and has an E color grade, which in most cases would be a positive feature that adds to its value.

marquise cut diamond with strong fluorescence

However, here’s where fluorescence comes into the picture. In a diamond with a relatively low color grade, a faint blue fluorescence may enhance its appearance by making it appear whiter to the human eye.

In a diamond with a high color grade, like the one shared by this reader, fluorescence doesn’t offer this benefit. Instead, a diamond with a color grade of G or better may look cloudy when it has medium or strong fluorescence. 

This is one of those situations where dropping down a few color grades (for example, choosing an H color diamond) would be a good idea. A diamond in this range with medium fluorescence will actually benefit from the fluorescence. 

We ended up suggesting this diamond from James Allen to the reader. It’s a 1.03 carat, H color, SI2 clarity marquise diamond. Unlike the original diamond, which had strong blue fluorescence, this diamond has no fluorescence at all.

marquise cut diamond with no fluorescence

So, why is this diamond, with a lower color grade and a lower clarity grade, a better choice than the original diamond? 

The first reason is that its color is absolutely fine for a marquise diamond. As we’ve explained in our guide to diamond color, there’s no reason to overpay for a diamond with a color grade of D, E or F. Instead, it’s better to choose a diamond that will appear white in relation to its setting.

When it comes to marquise diamonds, the visible difference in color between a G and an H, for example, is impossible to see without bright studio lighting and magnification. Set in a ring, both diamonds will look beautiful on a person’s finger. 

For marquise diamonds, we generally recommend aiming for a color grade in the G to H range, as long as it appears colorless to your eye. Beyond this level, you’re paying extra for something you’ll never notice. 

The same is true for diamond clarity. The marquise cut is a modified brilliant cut, with 56 to 60 facets and great light performance. As such, it’s very good at hiding inclusions that may be more visible on a less brilliant diamond. 

This is especially true for inclusions around the edges of a marquise diamond, which are often hidden behind the prongs that secure the corners. 

When it comes to clarity, it’s much more important to visually inspect a diamond and check that it’s eye-clean than to simply rely on its certificate. A lot of the time, a good SI1 diamond can look equally as clean to the eye as a VS1 or VS2. 

By focusing on what you can see and minimizing the amount you spend on features you’ll never see, you can get a larger diamond or more beautiful setting for your money. This is an important factor when you’re looking for an engagement ring in the $3,000 to $5,000 range. 

In the end, this reader ended up going with the 1.03 carat, H color, SI2 clarity marquise diamond we suggested and paired it with a beautiful pavé setting. His fiancée was ecstatic, which is what ultimately matters the most. 

Our Recommendation

It’s easy to focus solely on the 4Cs of diamonds and pass over things like fluorescence. This is because a diamond’s cut quality, carat weight and clarity are all easy to judge in photos, while fluorescence is often hard to assess without seeing a diamond in person. 

Ultimately, it’s up to your own tastes to determine if fluorescence is a problem. If you’re worried about fluorescence, you may want to stick with diamonds that either have no fluorescence at all, or faint fluorescence that doesn’t impact the color (or even improves it).

One of many advantages of buying a diamond online is that you have access to excellent return policies. Both James Allen and Blue Nile provide no questions asked return policies, giving you the ability to return a diamond if it looks milky when you see it in real life. 

Since fluorescence can be difficult to judge on your own, feel free to contact us for personalized help. We select hundreds of diamonds for readers every month and can help you find the ideal diamond and setting for your taste and budget.

James Allen James Allen is the leader in online diamond sales. Their imaging technology is the same as inspecting a diamond with a jeweler's loupe. They have the largest exclusive loose diamond inventory online and fantastic prices. They also have the nicest collection of lab-created diamonds online.
What we love about them:
  • No questions asked returns within 30 days of shipment. James Allen will send you a paid shipping label to return the ring.
  • Lifetime Warranty
  • Free International Shipping
  • Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
  • Provide insurance appraisals
  • One free resizing within 60 days of purchase
  • Free ring inscriptions
  • Best-in-class high quality imagery of all diamonds in stock
  • 24/7 Customer Service
  • Best-in-class packaging
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Blue Nile Blue Nile is the largest and most well-known internet jewelry seller. They have a very large exclusive online inventory. Their high-quality images are catching up to James Allens' and their prices are amazing. 
What we love about them:
  • No questions asked returns within 30 days of shipment. Blue Nile will send you a paid shipping label to return the ring.
  • Lifetime Warranty
  • Free Shipping
  • Free prong tightening, repolishing, rhodium plating and cleaning every 6 months
  • Provide insurance appraisal
  • One free resizing within the first year of purchase
  • High quality images of about half of their diamonds
  • 24/7 Customer Service
  • 100% credit towards future upgrades (must be at least double in value)
  • Best in class fulfillment
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