The 4Cs of Diamonds Explained: A New Zealand Buyer's Guide

Cut — The Most Important C

Cut refers to how well a diamond's facets interact with light. It is not the same as shape. A round diamond and an oval diamond have different shapes, but each can be cut anywhere from Excellent to Poor. Cut is graded on five levels — Excellent, Very Good, Good, Fair, Poor — and it is by far the biggest driver of how much a diamond actually sparkles.

A poorly cut diamond leaks light through the bottom or sides, and looks dull no matter how flawless the other three Cs are. An Excellent-cut diamond returns light straight back up through the top, producing the brilliance and fire most people associate with a beautiful stone.

Our recommendation for NZ buyers: never compromise on cut. If you have to trade something off, trade colour or clarity — not cut. The visual difference between an Excellent and a Good cut is dramatic; the difference between a G and an H colour is almost invisible.

Colour — The D-to-Z Scale

Diamond colour is graded from D (completely colourless) to Z (visibly yellow or brown). Most people cannot tell the difference between a D and a G with the naked eye. Below H, a faint warmth begins to appear, and from K onwards, the colour becomes obvious in good lighting.

The metal you pair the diamond with changes the equation. A G or H stone looks bright white in a platinum or white gold setting. In yellow or rose gold, the surrounding metal masks faint warmth in the diamond, so you can drop comfortably to J or even K without anyone noticing — and save meaningfully on price.

The NZ sweet spot: G–H for white metals, J–K for yellow metals. Going higher than G is mostly paying for a grade no one will see.

Clarity — How “Clean” the Diamond Is

Almost every diamond contains tiny natural marks called inclusions — microscopic crystals, feathers, or clouds formed during the diamond's growth. Clarity grades how visible these marks are under 10x magnification, on a scale from FL (flawless) down to I3 (heavily included).

Here is the secret most jewellers will tell you in person: anything graded VS2 or better is almost always “eye-clean,” meaning you cannot see any inclusions without a loupe. SI1 is often eye-clean too, depending on where the inclusions sit. Paying for VVS or FL is rarely worth it for an engagement ring.

The NZ sweet spot: SI1 to VS2. You get a stone that looks identical to a flawless one, at a fraction of the cost.

Carat — Weight, Not Size

A carat is a unit of weight, not size. One carat equals 0.2 grams. Two diamonds of the same carat weight can look very different in size depending on how they are cut — a deep-cut stone hides weight underneath, while a well-proportioned stone shows its full diameter from the top.

Diamond prices do not climb smoothly with weight — they jump at “magic sizes”: 0.50ct, 0.70ct, 1.00ct, 1.50ct, and 2.00ct. A 0.95ct diamond is often 25–30% cheaper than a 1.00ct of the same quality, even though the size difference is invisible. Looking just below a magic size is one of the easiest ways to stretch an NZ engagement ring budget.

Average centre stone size for engagement rings in New Zealand sits between 0.70ct and 1.20ct, depending on shape and budget. A halo setting can make a 0.70ct centre stone look closer to 1.00ct on the finger — a useful trick we cover in our ring styles guide.

Which C Matters Most by Budget

How the 4Cs Interact in Real Life

The 4Cs are not independent dials — they work together. A stunning SI1, G, Excellent-cut diamond will outperform a VS2, D, Good-cut diamond at the same price, every time. The first stone has the qualities you actually see; the second is paying for grades hidden beneath an inferior cut.

Our advice for NZ buyers is to think of the 4Cs as a budget. You have a fixed amount to spend — where you spend it inside the diamond matters more than the total. Spend on cut first. Spend on carat second. Save on colour and clarity by going to the threshold where the eye can no longer tell the difference. That is the value most jewellers will quietly direct you toward when you are sitting across from them.

If you want to go deeper into how lab-grown diamonds change the 4C calculation, read our guide on lab-grown vs natural diamonds in New Zealand. Lab-grown stones often let you go up two full grades on every C for the same budget.