Prioritise an Excellent or Very Good cut, which drives sparkle; choose G–H colour and eye-clean VS2–SI1 clarity for the best value; pick a size in mm that suits the finger. Stones of 3.6 mm and above should come with a certificate (GIA or a Vietnamese lab) whose number matches the laser inscription on the stone.
What are the 4Cs?
The 4Cs are the international criteria for grading a diamond: Cut, Color, Clarity and Carat. Popularised by the Gemological Institute of America (GIA), they are now the common language of the trade. Together they determine beauty and price; no single C decides everything.
Cut: the most important factor
Cut is not the shape (round, oval…) but the quality of the facets: proportions, symmetry and polish. A well-cut stone returns light to the eye as brightness, fire (rainbow flashes) and sparkle. A poorly cut stone looks dull and smaller, even with high colour and clarity.
| Cut grade (GIA) | Assessment |
|---|---|
| Excellent | Optimal light return; recommended for engagement rings |
| Very Good | Very beautiful, softer price than Excellent |
| Good | Acceptable on a tight budget |
| Fair, Poor | Lacks sparkle; not recommended |
For round brilliants, also check Polish and Symmetry on the report; ideally Excellent for Cut, Polish and Symmetry ("3EX").
Colour
White diamonds are graded from D (completely colourless) to Z (light yellow or brown). Adjacent grades are hard to tell apart by eye, but prices differ significantly.
| Group | Grades | Advice |
|---|---|---|
| Colourless | D, E, F | Top tier, highest price |
| Near colourless | G, H, I, J | Best value: looks white once set |
| Faint colour | K, L, M | Pairs well with yellow or rose gold |
Pairing tip: G–H stones look very white in white gold; I–J stones look harmonious in yellow or rose gold, which masks the faint tint.
Clarity
Most natural diamonds have tiny internal features (inclusions) or surface marks (blemishes). Clarity grades their number, size and position under 10× magnification.
| Grade | Meaning |
|---|---|
| FL, IF | No inclusions visible (IF: only tiny surface marks); very rare |
| VVS1, VVS2 | Minute inclusions, hard for experts to see at 10× |
| VS1, VS2 | Minor inclusions seen at 10×, invisible to the eye |
| SI1, SI2 | Noticeable at 10×; many SI1 stones are still eye-clean |
| I1, I2, I3 | May be visible to the eye, can affect brilliance |
The sweet spot for most buyers is eye-clean VS2–SI1: no visible flaws at normal viewing distance. View the actual stone or a magnified photo before choosing SI.
Carat and size in millimetres
Carat is weight: 1 carat = 0.2 g. In Vietnam, round diamonds are often described by diameter in mm (*ly*), such as "4.1 mm" or "5.4 mm". Approximate conversion for well-cut round stones:
| Diameter (mm) | Approx. weight (ct) |
|---|---|
| 3.0 | 0.10 |
| 3.6 | 0.17 |
| 4.1 | 0.25 |
| 4.5 | 0.33 |
| 5.0 | 0.45 |
| 5.4 | 0.60 |
| 6.0 | 0.80 |
| 6.5 | 1.00 |
| 7.4 | 1.50 |
| 8.2 | 2.00 |
Reference values; actual weight depends on each stone’s proportions.
Prices jump at round weights such as 0.50 ct and 1.00 ct. A 0.90–0.95 ct stone looks almost like 1 ct but costs noticeably less.
On slim fingers, 4.5–5.4 mm already stands out; larger fingers may suit 5.4–6.5 mm.
Diamond certificates: GIA and local labs
A certificate (report) is an independent assessment of the stone. GIA is the most widely recognised lab in the world; there are also Vietnamese labs. At Ngọc Diệp Gold Shop, diamonds of 3.6 mm and above come with a 4C certificate.
On a GIA report, read:
- Report number: often laser-inscribed on the girdle; verify it on GIA Report Check.
- Shape and Cutting Style, Measurements.
- Carat Weight, Color Grade, Clarity Grade, Cut Grade.
- Polish, Symmetry: finish quality.
- Fluorescence: glow under UV (None, Faint, Medium, Strong, Very Strong).
- Clarity characteristics / plotting diagram: where inclusions are.
When collecting your ring, ask to see the girdle inscription under a loupe and match it to the report.
Is fluorescence bad?
About a third of natural diamonds fluoresce (usually blue) under UV. None or Faint does not affect appearance. Medium to Strong can make I–J stones look whiter, but in a few high-colour stones may look slightly hazy. If possible, view the stone in daylight before deciding.
Shapes and settings
- Round brilliant: the most sparkle, most popular, holds value well.
- Oval, pear, marquise: look larger than a round of the same weight and lengthen the finger.
- Princess, cushion, emerald: modern or vintage styles.
Settings: 4 prongs show more stone and look larger; 6 prongs hold more securely; a bezel is safest for active lifestyles; a halo of small stones makes the centre look bigger. Mountings are usually 750 gold so the prongs are hard enough.
Getting the best stone for your budget
- Prioritise an Excellent or Very Good cut.
- Drop colour to G–H (or I–J in yellow gold).
- Choose eye-clean VS2–SI1 clarity.
- Buy just under round weights (0.45 instead of 0.50; 0.90 instead of 1.00).
- Consider a halo or oval shape to look larger.
- Always ask for a certificate from 3.6 mm and about trade-in policy.
Diamond trade-in
Resale prices are lower than purchase prices because retail includes cutting, certification, mounting and distribution. At Ngọc Diệp, diamond settings carry a 1-month warranty; sell-back is at a 25% deduction and exchange at 20%, based on the invoice price (per the published policy). Keep your invoice and certificate. See How jewellery is priced and traded in.
Quick answers
Which of the 4Cs matters most?
Cut, because it decides brightness and sparkle; a poorly cut stone looks dull even with high colour and clarity.
How many carats is a 4.1 mm diamond?
A well-cut round 4.1 mm diamond weighs about 0.25 carat.
Which colour and clarity give the best value?
For most buyers, G–H colour and eye-clean VS2–SI1 clarity look excellent at a sensible price.
How do I know a GIA report is genuine?
Check the report number on GIA Report Check and match it to the laser inscription on the girdle.
Do Ngọc Diệp diamonds come with certificates?
Yes. Diamonds of 3.6 mm and above have a 4C certificate (GIA or a Vietnamese lab).
What gold should a diamond be set in?
Italian 750 (18K), hard enough to hold prongs; white gold makes stones look whiter, yellow or rose gold suits I–J stones.
