Round
Search round diamonds → Excellent Acceptable Avoid
Crown angle34–34.9°
Pavilion angle40.6–41°
Avoid: table below 50% or above 69%, depth below 56.5% or above 65%
Updated October 2026 · By Fady Sawaya, 40 years in diamonds
Diamond buying guide: how to choose a diamond
This diamond buying guide gives you the numbers that actually decide how a diamond looks: the cut proportions for each shape, how much colour and clarity you need at each carat size, and the red flags that should make you walk away. It is written from 40 years of buying and setting diamonds in Montreal.
Every diamond on Xact3D also gets a score out of 100 built on these same rules. At the end of the guide, you can enter any certificate and check a diamond yourself.
01 · Step by step
How to buy a diamond in 6 steps
- 01
Set your budget first.
Decide what you want to spend, then fit size and quality inside it.
- 02
Choose the shape.
Shape changes everything else: the ideal proportions, how much colour shows and how visible inclusions are.
- 03
Put cut first.
A well-cut diamond looks bigger and brighter than a heavier, poorly cut one. Use the proportion ranges below.
- 04
Buy colour and clarity for the size.
An H that looks white at 0.5 ct can look warm at 2 ct. Aim for eye-clean, not flawless.
- 05
Check the certificate.
Buy only stones graded by a recognized lab, and match the report number to the stone.
- 06
See the actual stone.
Numbers filter out bad stones; a video in daylight confirms the good ones.
02 · The 4Cs
The 4Cs: what matters most when buying a diamond
The 4Cs are cut, colour, clarity and carat weight. Carat sets the size and much of the price. The other three decide how beautiful the stone looks, and they don’t count equally: cut matters most. That is why the Xact3D Score weighs them like this:
Cut 45% Colour 30% Clarity 25%
Cut. How well the stone handles light. It matters most, because a well-cut diamond looks bigger, brighter and livelier.
Colour. How white the stone looks for its size and shape. Larger stones and fancy shapes show colour more.
Clarity. Whether inclusions are visible, judged against what you can actually see at that size.
03 · Cut
Diamond cut: the numbers to look for on a certificate
Table %
The flat top facet, as a percentage of the stone’s width. Too big and the stone loses fire (the coloured flashes). Too small and it looks smaller and less bright.
Depth %
Total height divided by width, the most important number for fancy shapes. Too deep and the diamond hides its weight underneath, looks smaller and often shows a dark “bow-tie” across the middle. Too shallow and light leaks out the bottom, leaving a glassy “window”.
L/W ratio
Length divided by width. It sets the outline: how slim an oval is, how long a pear is. Partly a matter of taste, but the extremes look stubby or stretched.
Girdle
The thin edge around the widest point. Extremely thin edges chip easily; very thick ones add weight (and price) you can’t see from the top. It counts when the certificate reports it.
Polish & symmetry
How precisely the facets were finished and aligned. Look for Excellent or Very Good. On fancy shapes, good symmetry keeps the outline even and the sparkle balanced.
Culet
The tip at the bottom. We show it but don’t count it: most lab-grown certificates leave it out, and step cuts (emerald, asscher) end in a line rather than a point.
Ideal proportions for each diamond shape
Each shape is cut differently, so the ideal numbers change. These are the ranges that earn our top “Excellent” rating, and the point where a stone starts to look clearly worse.
Oval
Search oval diamonds → Excellent Acceptable Avoid
Avoid: table below 50% or above 70%, depth below 50% or above 74%
Pear
Search pear diamonds → Excellent Acceptable Avoid
Avoid: table below 50% or above 70%, depth below 50% or above 74%
Marquise
Search marquise diamonds → Excellent Acceptable Avoid
Avoid: table below 50% or above 70%, depth below 50% or above 74%
Heart
Search heart diamonds → Excellent Acceptable Avoid
Avoid: table below 50% or above 70%, depth below 50% or above 74%
Princess
Search princess diamonds → Excellent Acceptable Avoid
Avoid: table below 53% or above 85%, depth below 56% or above 84%
Cushion
Search cushion diamonds → Excellent Acceptable Avoid
Avoid: table below 54% or above 73%, depth below 54% or above 73%
Emerald
Search emerald diamonds → Excellent Acceptable Avoid
Avoid: table below 51% or above 79%, depth below 54% or above 79%
Asscher
Search asscher diamonds → Excellent Acceptable Avoid
Avoid: table below 51% or above 79%, depth below 54% or above 79%
Radiant
Search radiant diamonds → Excellent Acceptable Avoid
Avoid: table below 51% or above 79%, depth below 54% or above 79%
04 · Colour & clarity
Diamond colour: how white is white enough?
Colour is graded from D (colourless) to Z (light yellow). Neighbouring grades differ very little; what matters is whether you can see warmth face-up. That depends on two things:
Size. A small stone hides colour. The same H that looks white at 0.5 ct can look slightly warm at 2 ct.
Shape. Round brilliants hide colour best. Fancy shapes gather it at their tips, and step cuts (emerald, asscher) show it through their large open facets.
So we rate a colour grade against the stone’s size and shape.
Diamond clarity: buying eye-clean
Clarity runs from FL (flawless) to I3 (heavily included). What matters for beauty is whether you can see anything without magnification. A well-placed SI1 can look exactly like a VVS1 from a normal viewing distance.
Larger stones have bigger facets, so inclusions are easier to spot, and step cuts are a grade stricter again because their open facets act like windows.
What to aim for, by shape and size
Pick a shape and a carat size to see the colour and clarity grades that score Excellent and Very Good.
Shape
Carat
Colour
ExcellentD–F
Very GoodG–H
Fancy shapes gather colour at their tips.
Clarity
ExcellentFL–VS1
Very GoodVS2–SI1
A well-placed SI1 can look like a VVS1 at a normal viewing distance.
Show the full charts
| Colour | under 0.50 ct | 0.50–0.99 ct | 1.00–1.99 ct | 2.00 ct and up |
|---|---|---|---|---|
| Round | D–GH–I | D–GH–I | D–FG–H | D–FG |
| Oval, pear, marquise, heart, princess, cushion, radiant | D–GH–I | D–FG–H | D–FG–H | D–FG |
| Emerald, asscher | D–GH–I | D–FG | D–FG | D–FG |
| Clarity | under 0.50 ct | 0.50–0.99 ct | 1.00–1.99 ct | 2.00 ct and up |
|---|---|---|---|---|
| Round and brilliant shapes | FL–VS2SI1–SI2 | FL–VS1VS2–SI1 | FL–VVS2VS1–VS2 | FL–VVS2VS1–VS2 |
| Emerald, asscher | FL–VS2SI1 | FL–VS1VS2 | FL–VVS2VS1–VS2 | FL–VVS2VS1 |
Fluorescence
A blue glow under UV light, usually harmless. Strong or very strong fluorescence can make a top-colour stone look slightly hazy, so it costs a few points. On I, J and K stones, blue fluorescence makes them look whiter, so there it costs nothing.
Tint
Some stones carry a brown, grey or green tint the colour grade doesn’t show. When the supplier reports one, we take points off: up to 18 for green, 12 for brown or grey.
Eye-clean
When the supplier confirms a stone is eye-clean, it earns 5 extra points. A stone reported as not eye-clean loses 20.
Milky
A cloudy, hazy look that dulls the sparkle. Light milkiness costs 10 points, medium 20, and a heavily milky stone is rated Poor for clarity.
05 · Carat
Carat weight: size, price and magic numbers
Carat is weight, not size: one carat is 0.2 g. Two 1 ct diamonds can face up very differently. A stone cut too deep hides weight underneath and looks smaller, so a well-cut 0.90 ct can look as big as a poorly cut 1.00 ct.
Prices jump at round weights such as 0.50, 1.00, 1.50 and 2.00 ct. A stone just under, like 0.90 to 0.99 ct, can cost noticeably less and look the same once set. Compare diamonds by their measurements in millimetres, not only by carat.
Buy just under the magic numbers
0.50 ctTry 0.45–0.49
1.00 ctTry 0.90–0.99
1.50 ctTry 1.40–1.49
2.00 ctTry 1.90–1.99
06 · Natural or lab-grown
Natural vs lab-grown diamonds
Lab-grown diamonds are real diamonds: same carbon, same hardness, same sparkle. The difference is origin and price. A lab-grown stone costs much less for the same size and quality, while a natural diamond is rarer and holds its value better over time.
Natural
Rarer, and holds its value better over time.
Graded mainly by GIA, AGS, HRD and GCAL.
Lab-grown
Same carbon, hardness and sparkle, for much less at the same size and quality.
Graded mostly by IGI; we score lab-grown against lab-grown.
Every rule in this guide applies to both. One difference: lab-grown stones are mostly graded by IGI, so the Xact3D Score compares lab-grown stones with other lab-grown stones. More about lab-grown diamonds →
07 · Certificates
How to read a diamond grading certificate
A grading report is the diamond’s identity card. Before you buy, check:
Lab. We take GIA, AGS, HRD and GCAL grades as printed. IGI is widely used and reliable but grades natural stones a little more generously, so natural stones graded by IGI lose 3 points. Lab-grown stones are compared with other lab-grown stones, where IGI is the standard.
08 · Red flags
Diamond red flags: when to walk away
Depth over ~70% on an oval, pear, marquise or heart
Weight hidden underneath. The stone faces up small and often shows a strong dark bow-tie.
Depth under ~56% on any fancy shape
Too shallow. Light passes straight through, leaving a washed-out “window” in the centre.
Extremely thin or very thick girdle
Thin edges can chip when the stone is set; thick ones hide carat weight you pay for but can’t see.
A milky or tinted stone
Brown, grey, green or hazy stones look dull whatever their grade says. Ask for a video in daylight.
Low clarity in a step cut
Emerald and asscher cuts show inclusions clearly. Below VS2 at 1 ct and up, look at the stone closely.
09 · Try it
Check a diamond with the Xact3D Score
Copy the values from the certificate. Leave a box empty if it isn’t reported, and see what that does to the score.
A weak area pulls the score down more than strong areas push it up, so a dark, badly cut stone can’t hide behind a perfect colour grade. If any part is rated Poor, the overall score can’t go above 59.
No score? Some certificates leave out details we need, such as the depth or the polish grade. We still rate every value that is there, but we don’t give an overall score from incomplete data, and we tell you what is missing. Rare shapes (old European cuts, rose cuts, kites…) and fancy-colour diamonds are not scored.
The Xact3D Score is our own assessment, based on our experience as diamond wholesalers and on published cut, colour and clarity guidelines. It is not a laboratory grade. Always rely on the diamond’s grading certificate.
10 · In Canada
Buying a diamond in Canada
Diamonds trade worldwide in US dollars, so a price that looks low online can change once exchange, shipping, insurance and import costs are added. Ask for the full price delivered in Canada before you compare.
If Canadian origin matters to you, look for a stone in a Canadian origin program such as CanadaMark, which traces the diamond back to the mine.
For jewellers
Xact3D supplies natural and lab-grown diamonds to jewellers across Canada, with the stone, shipping and insurance in one price in Canadian dollars. Share this guide with your clients, and search our diamonds when you’re ready to source.
11 · Questions
Diamond buying FAQ
What is the most important C when buying a diamond?
Cut. It decides how much light the diamond returns, so a well-cut stone looks bigger and brighter than a poorly cut one of the same weight.
What colour and clarity should I buy?
For most round diamonds under 1 ct, G–H colour and VS2–SI1 clarity look white and clean to the eye. Larger stones and step cuts need higher grades; see the charts above.
Is a lab-grown diamond a real diamond?
Yes. Lab-grown diamonds have the same chemistry, hardness and sparkle as natural diamonds; they cost less and are graded on the same scale.
Which diamond grading lab is best?
GIA is the reference for natural diamonds. AGS, HRD and GCAL are also strict. IGI is reliable and the standard for lab-grown stones, but slightly more generous on natural ones.
What does “eye-clean” mean?
No inclusions are visible without magnification at a normal viewing distance. A well-placed SI1 can be eye-clean; a VS2 emerald cut sometimes isn’t.
What depth percentage is best for an oval diamond?
About 58–62%. Above about 70%, the stone faces up small and often shows a dark bow-tie; below about 56%, it can look glassy.
Should I buy a diamond just under 1 carat?
Often, yes. A 0.90–0.99 ct diamond can cost noticeably less than a 1.00 ct and look the same once set.
Numbers are a filter, not a verdict.
They reliably catch badly cut stones, but two diamonds with the same numbers can still look different. Before you buy, look at a video of the actual stone, or ask us and we’ll review it for you.