Spada Jewellers — Education
Diamond Guide
Everything you need to understand, select and sell a diamond — from the 4Cs to lab-grown certification.
The 4Cs
Every diamond is assessed on four criteria: Cut, Colour, Clarity and Carat weight. Together they determine a diamond's quality, appearance and price. Cut has the biggest visual impact — a poorly cut stone of high colour and clarity will still look dull.
How They Interact
- Cut first. An Excellent cut in G/VS2 will outshine a poor cut in D/FL. Never sacrifice cut to upgrade colour or clarity.
- Colour before clarity. Colour is visible across the face-up stone. Most clarity inclusions are only visible under 10× magnification.
- Sweet spot: Excellent cut · G or H colour · VS2 or SI1 clarity delivers maximum visual quality per dollar.
- Carat vs spread. A 0.95ct stone can appear identical in size to a 1.00ct stone but costs significantly less.
When a customer has a fixed budget, the order to compromise is: first clarity (stay eye-clean), then colour (G–H reads white in most settings), never cut. A well-cut H/SI1 will always beat a poorly cut D/FL in person.
Diamond Cut
Cut is the only one of the 4Cs determined entirely by human skill. It measures how well a diamond's proportions, symmetry and polish interact with light to produce brilliance, fire and scintillation.
Cut Grades — Impact on Brilliance
The Three Components
Key Proportions (Round Brilliant)
| Measurement | Excellent | Very Good | Acceptable |
|---|---|---|---|
| Table % | 54–57% | 52–62% | 47–69% |
| Depth % | 61–62.5% | 59–63.5% | 57–65% |
| Crown Angle | 34–35° | 32–36° | 29–38° |
| Pavilion Angle | 40.6–41° | 40.4–41.5° | 40–42° |
| Girdle | Thin–Med | Very Thin–Slightly Thick | Very Thin–Thick |
- Cut only applies as a formal grade to round brilliant diamonds. Fancy shapes are graded on Polish and Symmetry only.
- A deep stone (high depth %) "leaks" light through the bottom — it looks smaller than its carat weight suggests.
- A shallow stone (low depth %) loses light through the sides — the centre looks dark or glassy.
Spada sources Excellent cut stones as standard. For round brilliants, specify Triple Excellent (EX EX EX) on Cut, Polish and Symmetry when briefing your stone request. The price difference over Very Good is minimal but the visual difference is significant.
Diamond Colour
Diamond colour is graded on the GIA/IGI D–Z scale, measuring the presence of yellow or brown tint. D is completely colourless. Z has a visible yellow hue. For most buyers, G–H offers the best value — near-colourless to the naked eye.
Colour Scale — D to J (Common Range)
Colour Groups
| Grade | Category | Visibility | Best Setting |
|---|---|---|---|
| D – F | Colourless | No tint detectable even by experts face-up | Platinum or white gold |
| G – H | Near Colourless | Tint invisible face-up; detectable face-down by experts | White gold or platinum |
| I – J | Near Colourless | Slight warmth detectable; reads white in most settings | Yellow or rose gold masks tint |
| K – M | Faint | Visible warmth face-up to trained eye | Yellow gold — tint blends |
| N – Z | Very Light–Light | Obvious yellow/brown tint | Not typical for white-metal settings |
For lab diamonds in white gold, G–H is the standard recommendation. For yellow or rose gold settings, I–J offers excellent value — the warm metal masks the slight tint and the saving can go to cut or carat.
Diamond Clarity
Clarity measures the presence of internal inclusions and surface blemishes. All grades are assessed under 10× magnification. What matters most to buyers is whether inclusions are visible to the naked eye — this is called being "eye-clean."
Clarity Scale
Eye-Clean Threshold
* SI1 is usually eye-clean; SI2 depends on inclusion type and position — review the plot/image before recommending.
Inclusion Types
VS2 is the value ceiling for most customers — there is no visible difference between VS2 and VVS1 to the naked eye, but the price difference is significant. SI1 is the budget sweet spot: eye-clean in most stones, with a considerable saving over VS. Always request the IGI plot image for SI1–SI2 before recommending.
Carat Weight
Carat is a unit of weight — 1 carat equals 0.2 grams. Larger diamonds are exponentially rarer, so price per carat rises sharply at key weights. Two diamonds of the same carat can appear different sizes depending on cut depth and shape.
Size Comparison — Round Brilliant (face-up diameter)
Carat to Millimetre Reference
| Carat | Round Ø (mm) | Oval L×W (mm) | Princess (mm) | Emerald L×W (mm) |
|---|---|---|---|---|
| 0.25 ct | 4.1 | 4.8×3.2 | 3.4 | 3.5×2.5 |
| 0.50 ct | 5.2 | 6.0×4.0 | 4.4 | 4.7×3.3 |
| 0.75 ct | 5.9 | 7.0×4.7 | 5.0 | 5.4×3.8 |
| 1.00 ct | 6.5 | 7.7×5.2 | 5.5 | 6.0×4.0 |
| 1.25 ct | 6.9 | 8.2×5.5 | 5.8 | 6.6×4.4 |
| 1.50 ct | 7.4 | 8.8×5.9 | 6.2 | 7.0×5.0 |
| 2.00 ct | 8.2 | 9.8×6.5 | 6.9 | 8.0×6.0 |
| 2.50 ct | 8.9 | 10.5×7.0 | 7.5 | 9.0×6.5 |
| 3.00 ct | 9.4 | 11.0×7.5 | 7.9 | 9.5×7.0 |
Price Jumps at Key Weights
- 0.50ct threshold: Stones just below (0.45–0.49ct) sell at a significant discount to stones at or above 0.50ct.
- 1.00ct threshold: The biggest price jump in diamonds. A 0.95ct stone can look identical to 1.00ct but cost 15–25% less.
- 1.50ct and 2.00ct: Similar jumps — buying just below (1.45ct, 1.90ct) is the best value play for budget-conscious buyers.
- Spread matters: A "well-spread" 0.90ct can look larger than a deep-cut 1.00ct. Always check the mm diameter, not just the carat weight.
When a customer wants a "1 carat diamond" at a tight budget, offer a 0.90–0.95ct in Excellent cut — the face-up size is near-identical and the saving is real. For lab diamonds specifically, there is less stigma around going just-below, and the price differential is more pronounced.
Diamond Shape
Shape describes the overall outline of the diamond as viewed from above. Round Brilliant is the most popular and the only shape with a formal Cut grade. All other shapes are called "fancy cuts" and are graded only on Polish and Symmetry.
Shape vs Size — What Looks Largest
| Shape | Face-up Size (relative to round) | Notes |
|---|---|---|
| Marquise | +15–25% larger | Most surface area per carat |
| Oval | +10–15% larger | Popular elongated alternative to round |
| Pear | +10–15% larger | Depends on length-to-width ratio |
| Round | Baseline | Best light return, tightest supply |
| Cushion | Similar to round | Larger facets, softer look |
| Radiant | Similar to round | Strong brilliance for a fancy shape |
| Princess | Similar to round | Weight in corners — efficient yield |
| Emerald | –5–10% smaller | Elegant but less surface area |
| Asscher | –5–10% smaller | Same as emerald, square format |
Oval and marquise shapes deliver the most visual size per dollar — ideal when a customer wants impact within a tight budget. For step-cut shapes (emerald, asscher), recommend VS clarity or better as inclusions are more visible. For all fancy cuts, Spada sources on Polish and Symmetry — specify Excellent in both.
Lab-Grown Diamonds
Lab-grown diamonds are real diamonds — chemically, optically and physically identical to mined stones. They are created in a controlled environment rather than extracted from the earth, and are graded on the same 4C scale by the same gemological labs.
Lab vs Natural — Key Differences
How They Are Made
| Method | Full Name | Process | Common For |
|---|---|---|---|
| CVD | Chemical Vapour Deposition | Carbon-rich gas is superheated; carbon atoms settle onto a seed crystal layer by layer | Most white lab diamonds |
| HPHT | High Pressure High Temperature | Carbon is subjected to extreme pressure (~60,000 atm) and heat, mimicking deep-earth formation | Fancy coloured lab diamonds |
- CVD diamonds occasionally show graining or brown tints. High-quality CVD stones undergo post-growth treatment to remove this — look for high colour grades (E–F) in reputable lab reports.
- HPHT treatment is also applied to natural diamonds to improve colour — this is disclosed on the certificate. It is distinct from HPHT creation.
- A standard jeweller's loupe cannot distinguish lab from natural — specialist equipment (UV spectroscopy, photoluminescence) is required.
- All Spada stones are IGI certified and laser-inscribed — full traceability from report number to stone.
Lab-grown allows your customers to buy meaningfully larger or higher-quality stones at the same price point. A budget that buys a 0.70ct H/VS2 natural can buy a 1.80ct F/VS1 lab — in the same setting, to the same customer. Lead with value and quality, not origin.
IGI Certification
The International Gemological Institute (IGI) is the world's largest independent gemological laboratory and the standard for lab-grown diamond certification. Every Spada stone ships with a full IGI report.
What an IGI Report Contains
Verifying a Report
- Go to igi.world/report-check and enter the report number from the certificate or girdle inscription.
- The online report should match every detail on the physical certificate — carat, colour, clarity, cut, measurements.
- The laser inscription on the girdle is microscopic — visible under 10× loupe at the correct angle. Confirm the number matches the report.
- IGI issues a QR code on all recent reports — scan to verify instantly via the IGI app.
IGI vs GIA
| IGI | GIA | |
|---|---|---|
| Lab-grown standard | World leader | Growing |
| Natural diamond | Widely accepted | Industry gold standard |
| Report turnaround | Fast (1–3 days) | Slower (3–10 days) |
| Cost | Lower | Higher |
| Grading consistency | Slightly more liberal | Strictest standard |
| Spada stones | All stones certified | Available on request |
When a customer asks "is this certified?" — every Spada stone ships with a full IGI report, a QR code for instant verification, and a laser-inscribed girdle. The report number in our product listings corresponds directly to the physical report. Customers can verify their stone independently at igi.world at any time.