When it comes to diamonds in particular (though the same goes for virtually all gemstones too), traders refer to 'the four Cs' — cut, clarity, colour and carats (weight).
Diamonds and gemstones are cut in many different ways, with the choice of cut usually being based on what will most enhance the value of the stone; to recover the most weight from the rough stone, minimise the flaws and present its particular colour or brilliance best.
The table should not exceed 60% of the width of the stone and should be no smaller than 40%. Dark stones tend to have a large table to allow more light into the stone. Most modern gems cut by top professionals are done on a modern faceting machine or diamond-cutting machine, and the trend is to keep the table between 50% and 60% — 60% is the norm.
The pavilion main facets are crucial to the finished look of the stone and are cut at an angle that suits the density (specific gravity) of the stone. It's important to understand that diamonds and gemstones are cut to reflect light internally. Light coming in through the crown (or top) of the stone is gathered by the crown facets and reflected off the back pavilion facets and returned to the eye; this is why it's okay to set diamonds and gemstones in a rub-over or bezel setting — You don't need an open or claw setting to show off the brilliance of a stone.
The quality of the polish on facets (or cabochons) is also important for the brightness of a gem. Poorly polished facets will not allow enough light to go into the stone. A well-polished stone will bring out a depth and colour of cabochon stones. Also, poor polish will not allow light to reflect off facets, which, when highly polished and flat, act like small mirrors, reflecting light back out to the eye. Rounded facets don't do much for a cut stone unless it is a novelty type cut and facets are well polished.
Overall, the pavilion of a faceted stone does most of the work.
From a buyer's point of view, choosing a cut is more about the design of the finished ring. Baguette, marquise, oval and pear shapes, for example, can become dominant design features, so you need to be careful about buying something 'out of the ordinary' in the way of cut. A heart-shaped gemstone may look fabulous on its own, but designing it into a ring could prove difficult and expensive.
Poor cut and polish is seen often with Asian or 'native cut' stones. What is common with stones cut in Asia is that no regard is given to the laws of physics — economics usually rules in this instance and the stone is cut to the largest size or weight. If the stones are cut too shallow or deep for the density of that material, the stone will be dead or lifeless through the centre of the gem. Also, a stone cut to maximise the size of the table may have to sacrifice the depth of the stone, leading to a gem that is fragile, unbalanced and difficult to set. Gem Merchants and Jewellers call this effect a window; gem cutters call it a fish eye.
The bottom line is, do your homework, buy your gemstones and diamonds from a reputable dealer — one who has a passion for the stones they sell and an eye for quality. Finally, really look at the gemstone in every possible light, particularly down through the table where any flaws or cutting deficiencies will have the most impact on the finished ring.
Round Brilliant-cut: Most frequently used in diamonds, but also commonly used with other colourless gemstones, the round brilliant-cut ensures the maximum amount of light is reflected back through the table of the stone.
Step-cut: Typically used to show off coloured gemstones to their best advantage. In diamonds in can produce a bright, yet elegant, stone.
Mixed-cut: Often brilliant-cut above the girdle and step-cut below, the mixed-cut is most often used for coloured gemstones such as sapphires and rubies and other coloured transparent stones.
Fancy-cut: Refers principally to the overall shape — heart/pear/marquise and the like.
Cabochon cut: This produces a smooth-surfaced domed stone (no facets) in almost any shape. Rather than reflected light, it relies on the inherent colour of the stone.
The table below shows the most common cuts:
| Common Shapes | ||
| Round |  |
Usually brilliant-cut. |
| Square/Princess-cut |  |
Square stones are ideal for bezel or channel settings where you want the gem material flush with the mounting. |
| Trilliant |  |
Ideal for centre or side stones. Typically brilliant-cut. |
| Baguette |  |
Usually step-cut — also available as a tapered shape. |
| Emerald |  |
A step-cut variant. This cut offers protection to brittle material such as Emerald. |
| Oval |  |
Typically mixed-cut. Second most common cut after round brilliant. |
| Cabochon |  |
Round, oval or fancy — typically only coloured gemstones and semi-precious stones. |
| Fancy Shapes | ||
| Heart |  |
These are typically brilliant-cut or mixed-cut and are often used for very rare gemstones, irregularly shaped stones or to work around flaws. |
| Pear (Pendeloque) |  |
These are typically brilliant-cut or mixed-cut and are often used for very rare gemstones, irregularly shaped stones or to work around flaws. |
For both diamonds and gemstones, clarity refers to the absence or presence of flaws and impurities (or inclusions).
Generally, the fewer inclusions there are, the more valuable the stone — and this is especially so for diamonds. For some gemstones, such as emeralds, the presence of eye visible inclusions (described as 'Le jardin') does not detract from the value of the stone; some inclusions can help to establish origin. This is significant for the more expensive coloured material such as ruby and emerald where origin can add significant cost/value to a gemstone.
Inclusions are normal in most gem materials. With the exception of aquamarine, which should have no inclusions, there is no such thing as an inclusion-free (or flawless) diamond or gemstone.
It might be that a stone appears 'perfect' to the naked eye or even under low magnification, but under high magnification many natural stones will show at least some microscopic inclusions — even some synthetic gemstones - have characteristic inclusions.
Depending on what you're buying — and your partner's taste — basically you'll want either more or less.
For colourless stones (like white diamonds), obviously less colour is better.
In coloured stones — champagne and pink diamonds, sapphires, emeralds and so on — how much colour is desirable is more often than not a matter of personal taste.
Unless you're buying opal or parti (multi) coloured stones, what you really need to look for is even distribution of colour.
Some stones will show zoning when viewed from different angles — sometimes showing patches of virtually no colour — and this can really spoil the look of what would otherwise be a beautiful stone.
Carat is basically the weight of a diamond or gemstone. One standard metric carat is equal to 0.20 grams.
Within a type of stone, it can also serve as an indication of physical size, for example, a 1.00 carat diamond will be physically bigger than a 0.8 carat diamond of the same cut.
But this rule doesn't flow across different types of gem material.
Different gemstone materials have different densities (specific gravity). Hence a 1.00-carat sapphire will be a different physical size to a 1.00-carat diamond of the same cut.
You'll also often hear the term 'point'.
There are 100 'points' in 1 carat. A 10 point diamond, for example, is equivalent to 0.10 carats (or one-tenth of a carat).
Along with colour, clarity is one of the major criteria used in classifying diamonds. Generally speaking, the more 'clear' the stone, the higher its value.
The clarity of a stone principally relates to the visibility of inclusions and flaws, and it's only in low-grade diamonds that clarity becomes obvious.
This is important from a buyer's perspective. Choosing a lower grade diamond may enable you to buy a bigger stone for the same money — yet, depending on the location of the flaws, see no discernable difference in the appearance of the stone.
The difference will be reflected in the valuation of the ring, but if "bigger is better", sacrificing a grade or two in order to get a larger stone could be the way to go.
The most commonly used gem grading system is that developed by the Gemmological Institute of America (GIA).
In terms of visible flaws or inclusions, these classifications are based on what can be seen with a loupe (a magnifying glass of 10x magnification).
In the GIA system, there are 11 grades (from the top):
| FL | Flawless - No loupe-visible flaws internally or externally |
| IF | Internally Flawless - No loupe-visible flaws internally |
| VVS¹ VVS² |
Very, Very Small inclusion or imperfection that is very difficult to see under a loupe |
| VS¹ VS² |
Very Small inclusion or imperfection, difficult to see under a loupe |
| SI¹ SI² |
Small Inclusion or imperfection easily seen with a loupe |
| I¹ I² I³ |
Imperfect (or included) — Inclusions can be see with the naked eye |
| NB: The superscripts 1, 2 or 3 refer to the number and placement of visible inclusions. | |
The Scandinavian Diamond Nomenclature (Scan. D.N) and other systems vary slightly in their classifications, but the most obvious is in the I 1 to I 3 range where clarity is often referred to as P 1, P 2 and P 3 (where 'P' stands for Piqué — pronounced Pee-Kay).
From an appearance point of view, it's very difficult for an untrained person to distinguish between an FL and an SI² diamond. It's only when you get into the 'I' (or Piqué) grades that the imperfections start to become more easily visible.
If your budget dictates that you must go for a lower-grade diamond, one thing to be very mindful of with the 'I' or Piqué grade diamonds is the position of the flaw or inclusion.
If it's right on the edge of the girdle, then an inclusion could well be hidden by a claw or bezel when it's set — in which case the stone might appear perfectly good and you've got yourself a bargain.
If the imperfection is away from the girdle, somewhere in the middle of the stone, then it'll be visible.
As for clarity, when it comes to colour, less is (worth) more — until you get into 'fancy' coloured diamonds like pink and so-called champagne and cognac coloured diamonds.
Basically, 'white' diamonds vary in colour from colourless to very pale yellow — colourless being the most desirable.
The GIA scale for classifying diamond colour is shown below:
| Scale | Description |
| D E F |
Colourless |
| G H I J |
Near colourless |
| K L M |
Faint yellow |
| N O P Q R |
Very light yellow |
| S to Z | Light yellow |
| Fancy | Coloured |
Please note, G to J stones will not actually look 'yellow'. To an untrained eye, it is very difficult to see 'shades of colourless'.
Between diamonds of the same cut, say, round-brilliant, the carat weight of different stones can be used as a relative measure of physical size — given the proportions by which diamonds are cut are fairly uniform.
We all perceive the prestige attached to owning a 1.0 carat diamond ring (or bigger) — but just how big is a 1.0 carat diamond?
The following table will give you some idea of approximate physical size relative to weight (for round cut diamonds only):
| Carats/Points | Diameter |
| 2.00ct (200 points) | ~8.2mm |
| 1.50ct (150 points) | ~7.4mm |
| 1.00ct (100 point) | ~6.5mm |
| 0.75ct (75 point) | ~5.9mm |
| 0.50ct (50 point) | ~5.2mm |
| 0.25ct (25 point) | ~4.1mm |
| 0.10ct (10 point) | ~3.0mm |
Of course, this doesn't help you much when comparing stones of different cuts. A square or princess-cut diamond will look to be quite different in size relative to a round cut diamond of the same number of carats.
Colour and clarity are the two major determinants of the value of a diamond of any given weight (assuming it has been cut properly). Although proprietary cuts such as Hearts and Arrows TM and Lucida TM do attract a premium.
From a value perspective, given that inclusions in diamonds graded SI² and above are virtually invisible to the naked eye, in our opinion you should put colour before clarity when selecting from stones graded SI² and above.
Frankly, if you're on a budget, there's little to be gained by going above an SI¹ or SI² when it comes to clarity.
Broadly we recommend avoiding 'I' or 'P' stones of any weight or colour. Having said that, we are able to supply these stones and as with all our material our I and P stones are the best available. Our supplier only sources quality I and P stones, that is, whilst classified as I or P, their flaws are in the least conspicuous position (i.e. usually can be hidden by the setting).
For diamonds under 0.4 carats, you can get by with a G or H colour. The basic rule is: "Always buy the best you can afford".
Given that weight (or size) increases cost exponentially per stone, if you can't afford a single big diamond, consider purchasing several smaller stones.
Within the same clarity and colour grades, 'small' diamonds cost less per carat than 'large' ones.
By buying several small stones you can either get more carats for your money, or get the same total weight for fewer dollars.
When buying a diamond for your partner, you should be mindful of the size of her fingers.
The same sized diamond will look big on a woman with small hands and fingers, but small on a woman with large fingers.
Of course, for some people, to hell with it — just get the biggest possible — and that's fair enough.
But if bigger isn't necessarily better for you and your partner, you might consider buying a smaller diamond and spending the money you save on a platinum rather than gold ring — or an extra week away on your honeymoon!