Ideal Cut Diamonds Explained: Proportions to Maximize Light

Last Updated on September 25, 2022 by Juli "Jewels" Church

Wondering what an ideal cut diamond is, or how to buy one? You're in the right place.

In this Learning Guide, I'll go over all the ins and outs when purchasing ideal cut diamonds, and why they're worth the money.

I'll also answer the most popular questions about purchasing an ideal cut diamond like:

ideal cut diamond explained
  • Is an ideal cut diamond the same as an excellent cut diamond?
  • Can any diamond shape be an ideal cut diamond?
  • Are super ideal cut diamonds better than ideal cut diamonds?

What is an Ideal Cut Diamond?

An ideal cut diamond refers to a diamond that has been cut and faceted by diamond cutters to exact measurements and symmetry to achieve superior light performance. It is the best cut grade available on a diamond grading report or diamond certificate.

The Gemological institute of America invented the concept of cut grades, which are one of 4Cs of diamond quality. Diamond cut, diamond clarity, diamond color, and diamond carat weight are the main pillars that determines the overall quality of a loose diamond.

Diamond cut grades are broken down into 5 individual grades: Ideal/Excellent Cut, Very Good, Good and Poor. Most retailers sell Ideal Cut diamonds/Excellent Cut diamonds, Very Good diamonds and Good cut quality.


Buying an Ideal Cut Diamond

Just as with any purchase, there are downsides to choosing an ideal cut diamond for your engagement ring. But are these drawbacks dealbreakers? You be the judge.


  • Best diamond cut quality for your diamond
  • Exact proportions offer exceptional brilliance and fire
  • Comes with a diamond grading report


  • Ideal cut diamonds are expensive
  • Ideal cut diamonds appear smaller

Can All Diamond Shapes Be Ideal Cut Diamonds?

It may catch you off-guard to learn that only round cut diamonds can be true ideal cut diamonds. The reason for this is because round diamonds are the only diamond shape that can be cut down to exact proportions and symmetry. No other diamond shape can have perfect proportions.

On a diamond grading report, you won't see a cut grade for fancy diamond shapes. Fancy shapes are every other diamond shape beside a round diamond. You will see the numerical measurements and dimensions of the stone as well as the other 3cs.

If a retailer produces their own certificates and not ones from a gemological institute or laboratory, it may have a cut grade. I don't recommend buying ideal cut diamonds that have in-home certification anyway.

Ideal Proportions for Fancy Diamond Shapes

If only a round diamond can be an ideal cut diamond, then why can I find ideal cut princess cut diamonds and ideal cushion cut diamonds?

Great question. Let me explain.

Some retailers may organize their fancy shapes by the cut grade, but it really should be considered cut quality. SInce most online diamond retailers have sliders for cut grade filters, it just makes sense to do the same for all shapes. Fancy diamond shapes that are labeled "ideal" have ideal proportions, but not exact proportions.

Each diamond shape has its own ideal proportions for that shape. But because round brilliant diamonds are the only ones that can diamond cutters can produce with perfect proportions, there is a range for other diamond shapes.

Here is a chart that describes the ideal proportions for fancy diamond shapes. Note that even if you choose a diamond within the ideal proportion range, the grading report won't indicate any cut grade.

Recommended Ideal Proportions

Should I Buy an Ideal Cut Diamond?

When it comes to buying the perfect diamond, most of it is subjective. Yes, there's a standard on what a high quality diamond looks like and of course there's diamonds with the highest grades for all of the 4Cs. Unfortunately, most of us can't afford a diamond with the best grades in every category.

Fortunately, it's not really necessary to have the best diamond grades in everything. If you want to pay for them, go ahead. You'll end up with a pretty rare and valuable diamond. But for those of us who aren't made of money, it works out in our favor.

That being said, when it comes to your diamond cut quality, it's important to buy the best cut grade or quality you can get. If you're buying a round cut diamond and are able to afford an ideal cut diamond or excellent cut grade, I highly encourage it. Diamond cut is the only one of the 4Cs you shouldn't compromise in order to save money. You can always go down in carat weight, color grade, or clarity grade without compromising the overall quality.

The cut quality has a direct effect on your diamond's brilliance and fire. An ideal cut diamond reflects the most white light out of all the cut grades. It's not that Very Good diamond cut proportions won't have a beautiful sparkle, but an ideal cut diamond reflects nearly all the light that enters the diamond.

A poorly cut diamond results in what a professional gemologist would call light leakage. If the diamond's cut quality isn't up to par, white light entering the diamond can't reflect light back through the crown angle. Instead of reflecting back out, the light just enters the diamond and stops. Too much light leakage affects diamond sparkle, resulting in a dull and muted light performance.

A well cut diamond will still have some light leakage, but the diamond's brilliance and fire won't be impacted near as much. Often times, the average person may not be able to tell the difference between an Ideal cut diamond and a Very Good cut diamond cut grade. Others can.

I definitely urge you to buy an ideal cut diamond or a fancy shape with ideal proportions, but I realize not everyone can afford this. In this case, a Very Good cut grade or quality will still be a well cut diamond suitable for engagement rings.

Ideal Cut Diamond vs Excellent Cut Diamond Grade

On a grading report, you may notice that some gemological laboratories have Ideal cut grading or they have Excellent Cut as the diamond's cut grade instead. Most people in the diamond market and industry agree that an excellent cut quality is the same as ideal cut diamonds.

They are distinguished differently because GIA diamonds don't have Ideal cut grades. They have GIA excellent diamonds. The ideal cut grading system comes from the American Gem Society, which is probably the second most respected gemological laboratory. The AGS developed the cut quality called the AGS 0 Ideal cut diamond.

AGS Ideal Cut Grades

Some people argue which is better, a GIA excellent or an AGS Ideal cut diamond, but we think it's best left up to personal style and preference.

Ideal Cut Diamond vs Super Ideal Cut Diamond

You can also obtain super ideal cut diamonds, but they are more lot more money. They are also well cut diamonds with superior light performance, but most people wouldn't be able to tell the difference between super ideal cut and ideal cut diamonds.

A true super ideal cut diamond reflects more light return than an ideal cut diamond, but the difference is very minimal to the viewer's eye. If choosing between an ideal cut diamond and a super ideal cut diamond, you should compare ASET photos and Ideal Scope images. ASET photos show customers how well diamonds reflect light.

How Much Are Ideal Round Brilliant Cut Diamonds?

Because ideal and excellent diamond cuts have the most white light reflected and are the best cut grades, they're the most expensive. However, this is an expense you should make room for.

Round diamonds are the most popular diamond shape and the most expensive. A lot of people believe it's the most expensive shape because it's the most popular but that's not the case. A cushion cut diamond is the third most popular shape, and it's one of the most affordable ones in the diamond market.

Remember, the round brilliant is the only shape with equal and precise measurements. No other diamond's facets is able to produce that much light reflection.

ideal lab grown diamond

Another reason the price for these are higher than other diamonds is because more of the round diamond rough is discarded when being faceted. They only retain about 40% of the diamond rough and the rest is thrown out. A princess cut diamond retains about 70% of the rough and are more affordable.

The cost of an ideal cut diamond is dependent on multiple factors, not just its cut quality. The other 3Cs have an impact, as well as its polish and symmetry.

However, if you want an idea of the difference, here are two comparable diamonds from James Allen. The left diamond is the ideal cut diamond (excellent cut), and the right is a Very Good diamond. The ideal cut diamond is a true AGS 0 Ideal. The Very Good cut diamond has a GIA grading certificate

AGS Ideal Diamond
GIA very good diamond cut

How to Buy an Ideal Round Brilliant Cut Diamond

Whether you choose to buy your ideal cut diamond online or in person, there are a few things to keep in mind when looking for your perfect match.

  1. Don't buy an ideal cut diamond without a grading report from a reliable lab. For natural diamonds, we only recommend GIA or AGS certified diamonds. For lab grown diamonds, we recommend GIA, AGS, IGI, and GCAL diamonds.
  2. Remember that a round diamond looks smaller than other diamond shapes of the same carat weight. A round diamond's table size doesn't carry as much weight as other shapes. Instead, more weight is distributed in the pavilion, or bottom cone-shaped part of the round stone. A shorter crown height and deep pavilion allows the diamond to reflect and refract light back through the pavilion angle.
  3. If you're spending more of your budget on an ideal cut diamond, you may be able to save money on diamond clarity. The polish and symmetry of a round shape may be able to hide dark spots you may otherwise see in a diamond with a lower cut quality.


Buying an ideal cut diamond, whether it's a true AGS 0 Ideal or a GIA Excellent Cut is worth it if you're in the market for a round cut engagement ring. With other shapes, you should follow the ideal proportions for that shape. 

Ideal cut diamonds are more expensive than Very Good cut diamonds. You will have to determine if the cost difference is worth it to you. 

Personally, I vote yes, the cost is worth it. I'm a firm believer on go big or go home, and that doesn't necessarily have to do with carat weight. If I'm spending the money on a round cut diamond-the only diamond shape that has perfect measurements- you better believe I'm buying the top cut grade. I want to make sure I get my money's worth with superior brilliance and sparkle. 

The only reason I got into diamond jewelry in the first place when I applied to work at Kay's was because I liked sparkly things. I have my whole life. So yes please, give me all the sparkle. I'd gladly sacrifice carat weight or search for a lower eye-clean clarity grade. 

You might not think you're able to afford an ideal cut diamond. And that's okay. A Very Good cut diamond will do just fine. I don't advise you to buy below Very Good. The sparkle gets noticeably duller the lower you get. Go too low, and the durability of your diamond will be dull.

However, there's another option I urge you to consider before choosing a Very Good cut diamond. 

Did you know that lab grown diamonds are real diamonds that can cost 20-60% less than a mined diamond of the same grade? A lab grown ideal cut diamond is much more affordable than a mined natural ideal cut diamond. You may even be able to get a bigger ideal cut lab grown diamond than you would a smaller mined ideal diamond. 

The choice is yours. 

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