Choosing a beautiful diamond
By Garry Holloway
What makes a diamond beautiful and what makes it sparkle? GARRY HOLLOWAY offers advanced information for those with an existing understanding of diamond properties on how to choose a beautiful diamond. For a more basic run-down on selecting a diamond, click here: Buying A Diamond.
A diamond’s appearance is mostly a result of the way it has been cut and, unfortunately, “cut” is the Cinderella of the 4Cs; most diamonds are 'Cut for Carats' not for beauty. Anyone looking for a truly beautiful diamond will need to have a more in-depth knowledge so they learn to avoid some pitfalls.
Carat is the simplest and most objective of the 4Cs. Pop a diamond on the scales, if it weighs 0.2gm then it is a 1.00 carat diamond. A quarter carat is often called 25 points.
Everyone knows more carat weight means more cost. But what often surprises people is that BIG diamonds are very rare – double the weight costs around 4 times more. And the magic 1.00-carat weight, D colour flawless costs 1.7times more than a 99 point or 0.99-carat D flawless.
So for diamond cutters, reducing the carat weight to produce a smaller yielding Ideal Cut diamond is BAD for business. GIA surveyed 67,000 stones submitted for grading and found less than 3 per cent were Ideal Cut. But weight does not equal size!
These two diamonds have the same diameter, and each could be cut from the rough diamond in the centre. The dull tone on the left weighs more, so it sells for more.
Even when diamond cutters produce so-called Ideal Cuts, they usually can't resist leaving a little more weight on the crown and pavilion (the top and the bottom) to push the stone to the next “magic weight”.
This has a critical impact on a diamond’s beauty.
The magic weights are ½-carat, ¾-carat, 90 points, 1-carat, 1.5-carat, 2-carat, etc. “Under-sizes” are diamonds that weigh just below a magic weight; they are rare in ideal cut stones.
The girdle or edge thickness an important indicator of whether the diamond is well-cut or not. If a diamond has no girdle, or it is extremely thin, the diamond can chip easily. Medium to slightly thick is best, but thicker girdles add extra weight for no benefit and more cost.
Cutting transforms a diamond pebble into a sparkling gem. It is said that 98 per cent of a diamond's life and sparkle comes from its cut. But unlike carat, cut is complex and the least-understood of the 4Cs.
The bottom left picture shows a diamond that is cut too deeply, with light leaking out of the back. This is seen as white in the Ideal-scope (tm) image; the diamond looks dull and drab and has a smaller spread. Ideal cut diamonds, as seen bottom right, look red (with a star shaped black pattern).
Most diamonds have a hint of yellow or brown. The rarest and most expensive colourless diamonds are D or Icy white, on a scale that goes to Z and is yellowish. (More colour than Z is graded as a “fancy” colour). It is recommended that those wanting a whitish diamond choose a D to H colour because most people can easily detect the "off colour" in I and lower grades. Those wanting the very best should select a D to F "collection colour" as they are known in the trade.
Some people, however, actually like the "warmth" of I, J or K colours. But the main reason people buy a lower colour is simply to trade up in one of the other 4Cs. For instance in Asia, low clarity is synonymous with impurity and so colour is often traded-off for a higher clarity.
Usually about one in two people in a blind test can tell the difference between a D and an H coloured diamond; at I and lower, the majority of people can see the faint tint of yellow. Colour has a bigger impact on price as the clarity and carat weight goes up.
After carat weight, clarity has the biggest impact on diamond prices. Diamond clarity is symbolic of "purity" - the more flaws, the less valuable the diamond, but unlike emeralds, inclusions in diamonds are rarely "flaws" that result in breakage.
Medium clarity diamonds are just as brilliant as flawless diamonds; even experts cannot tell the difference between flawless and SI1 diamonds without a loupe.
SI2 is supposedly the borderline where inclusions become visible to one r naked eye. If one can see an inclusion with one r naked eye in normal light, from 14 inches (35cm) without having previously identified its position using magnification, then the stone is 'I' for Imperfect or the European term ' P' for pique (pronounced pee kay). However some one ng people with excellent eyesight may be able to spot a VS2 inclusion.
Those wanting the balance for quality and value are recommended to select an SI1 to VS2 stone.
Giving a grade based on inclusion quantity, size, placement and type is difficult; graders are human. Diamond grading reports are meant to resolve arguments between buyers and sellers, but ultimately they are just "expert opinions" under 10 times magnification with a loupe. Even the world’s leading diamond grading laboratory – the Gemological Institute of America, has given different grades for the same resubmitted stone.
Grading laboratory EGL and EGL-USA introduced an SI3 grade in 1992. SI3 has been used in the dealer market for many years because of the big price difference between I1 and SI2.
The World Federation of Diamond Bourses wants all labs to introduce SI3, but most refuse. Since 1992, most labs have softened their SI2 grades.
Grading reports include plots of inclusions (marked in red for internal and green for external features) and this is useful for identification. Often only the main "grade makers" are plotted, and additional inclusions are listed in comments: "pin points not shown" etc. A common comment is "Clouds Not Shown"; a cloud drawn on plots look so bad that no one would buy the diamond. Clouds are only a problem on SI1 and lower clarities if no other inclusion is marked on the plot – ie the cloud is the grade maker; a big cloud may dull the diamond. It is rare for even I1 diamonds to be dulled by inclusions.
WHAT TO BUY?
This authour cannot tell one what to buy, but personally buys the biggest, brightest, whitest, eye-clean diamond, with some blue fluoro: usually D-F, SI1. If you are sharp-eyed or detail-orientated, you might prefer higher clarity, but smaller size. Higher colour and higher clarity combinations cost a lot more than high colour and medium clarity or medium colour and high clarity. Most people can see the colour difference between I and D, very few can see the difference between IF and SI1. The buyer may not mind seeing an inclusion or two, or they may find low colour diamonds "warmer".
It is a good idea to view a range of diamonds to set one’s personal standards.
Fifty years ago only very rich people could afford big diamonds. They often bought high clarity on the recommendation of a trusted jeweller (who stocked on the advice of his trusted supplier). Today we see more sales of F-H VS2 to SI1 diamonds.
Demand for high-quality commercial grade stones (D-H, VS-SI) will probably continue to grow, and that will mean they will retain value and liquidity. Religion or culture is an important factor; in some cultures clarity = purity = divine powers bringing good luck.
Round brilliants are the most sparkly and most popular shape; therefore rounds have the highest liquidity should the owner ever need to resell or trade-up.
Various fancy shapes come and go in popularity (and rise and fall in value). Right now, for engagement rings, Princess cuts are hot. Princess cuts have more small sparkles than round diamonds; but below 1-carat, as we age, our ability to discern those tiny more frequent sparkles reduces. Princess cuts are more prone to chipping on the edges during wear and on the points during setting. To be forewarned is to be forearmed.
When buying a diamond over one carat, it is strongly recommend it be accompanied by an independent grading report (often called a certificate or cert). Grading reports don’t state how much a diamond is worth, they give an independent expert opinion on its quality and can only be issued for a loose stone. These grading institutes do not sell diamonds and the better, stricter labs are recognised worldwide. Diamonds with these reports trade for a few percent more.
But diamond grading is subjective, and therefore never 100 per cent consistent among even the top graders. So different reports on the same diamond, even from the same lab, may vary. Some Israeli labs are seen in the industry to be " soft labs".
Valuations go out of date, but unless the diamond is damaged, the cert is good forever. Store it in a safe place. Do not confuse a cert with a valuation. A valuation is an opinion of the value of the diamond or piece of jewellery and should be done by a registered valuer.
The attraction of fancy shapes is largely the appeal of the shape itself. Round brilliant cut diamonds are without doubt more brilliant. Generally speaking, larger fancy shapes cost a little less than round diamonds. This reflects the larger yield cutters get by cutting a fancy shape that “fits” into an unusual shaped piece of diamond rough.
There are no “ideal” fancies, but here are some tips. Expect to see a 'bow-tie effect (usually dark but can be lighter) in the centre of longer/narrower diamonds like marquise, pear and oval diamonds.
Popular length to width ratios are marquise 2:1, heart 1:1, princess 1:1 or square and emerald cut 1.5:1.
Watch out for very thick girdles that result in paying for excessive weight. Also beware of very or extremely thin girdles and especially thin points or ends on marquise and pears. Be especially wary of princess cuts with thin-girdled corners.
It is very easy to see inclusions and lower colour in emerald cut diamonds because they have a less “cluttered” look. The end facets are often very steep and can look very different to the side facets, it is better if they look similar.
CROWN AND PAVILION ANGLES
In 1992, the authour discovered an inverse relationship between a diamond's crown and pavilion angles (the top and bottom facet angles). Work with diamond cutters has given them new freedom to vary proportions to suit the rough diamond and achieve a beautiful gem. The shallow stone on the left has more light return and the steeper stone on the right shows more fire. This authour named them BIC - Brilliant Ideal Cut and FIC - Firey Ideal Cut.
The overlain profiles show how similar the light paths are, even though the angles vary considerably.
Many labs use a minimum–maximum crown and pavilion angle-based grading system that penalises cutters who produce such diamonds. Fortunately this has all now changed; the American Gem Society Lab has adopted the inverse proportion approach from June 1, 2005 . The GIA is likely to follow soon.
BRILLIANCE, FIRE and SCINTILLATION
Brilliance is an essential attribute of a beautiful diamond and has two components: brightness and contrast. Bright diamonds return lots of light from the surroundings back to a “face up” an observer. If light from above leaks out the back of a diamond, naturally it has less brightness. But light that enters and leaves in the face up direction is wasted because the wearer’s head blocks lights from that direction. Diamonds that are too deep or very shallow do this; they have areas that act like a mirror back to the viewer; they return less light and so they have less brightness.
But to be brilliant, a diamond needs more than just brightness from light return. Consider the contrast of a chessboard. Although it has only half the light return of a sheet of white paper, it appears brighter, especially when it is moved because it “scintillates”.
Fire or dispersed light appears as flashes of rainbow colours. One sees more fire in darker environments like restaurants that have just a few point light sources or a flickering candle.
Diamond experts have known for a long time that steep crown angles and small tables (like “old-cut” diamonds) produce more fire. But this combination also produces less light return. Less light return makes it easier to see firey flashes that might otherwise be swamped by bright white sparkles; that one reason is why old-cut diamonds and some fancy cuts appear to have a lot of fire.
Scintillation is the intense sparkles in a diamond as it moves. Black and white sparkles of scintillation show well in flood lit or office lighting environments where fire can be totally absent. Under pin-point or spotlights, fire also adds to scintillation. Ideally, a diamond has many pleasing flashes spread across the surface of the stone, with few dull dead patches.
Generally, the best looking diamonds have table sizes in the range of 55 to 60 per cent (measured as a percentage of the diameter of the diamond). The size of the table is more important in larger diamonds, say over ½-carat.
Variations in table sizes are less critical than crown and pavilion angles. Diamond cutters tend to cut larger table sizes than we would prefer because cutting larger tables conserves diamond weight.
Larger table diamonds have a better "spread" and can be more brilliant, ie – they return the most light, but have less fire and scintillation than diamonds with smaller tables.
This is because the ability of a diamond to break light into rainbow colours (fire or dispersion) is enhanced by light entering or leaving a diamond at an acute angle. The same principle applies to cut crystal wine glasses and chandeliers. One can probably visualise the Pink Floyd Dark Side of the Moon CD cover. Technically, dispersion is maximised as light approaches the critical angle between the diamond to air interface.
The resultant burst of colour emerges close to parallel to the surface of the diamond. So if one looks from the front of a diamond one are more likely to observe fire from a crown facet. The smaller the table the bigger the crown facets.
Large tables produce less scintillation because there is less interplay between the crown and pavilion facets. Scintillation is hard to define, it is the black - white - black flashing observed as the diamond is rolled (or the light source moved).
If a diamond had no crown facets at all it would appear very brilliant, but dull and lifeless.
One can see on the images here that the smaller table diamonds appear to have more facets than the larger table images. If a diamond had no crown facets it would appear boring.
Table sizes over 60 per cent are more affordable and because there is less crown height, they have a larger spread or diameter. One gets a bigger looking rock for less cash. An upper limit of 63 per cent is advisable.
One can see that Ideal-Scope images of larger table diamonds often look a little paler just inside the table; but what appears to be 50 per cent pink intensity will actually return 75 per cent or more light. As table sizes get larger, beware of the fish-eye effect.
FISH EYE EFFECT
A fish-eye is a nasty appearance that one can see just inside the table of a diamond. It looks slightly crazed and dull, just like a dead fish’s eye. The fish-eye is a reflection of the girdle (on the opposite side). If the girdle is not polished and is thick, the effect looks like a BIG circular inclusion, and can be as bad as an I3 (P3).
Fish-eyes are more apparent if the pavilion is shallow 39.5°, the table is large; the girdle is thick and not polished. Combinations of these factors worsen the effect. Fish-eyes occur between the following pavilion depths and table sizes: 41 degree pavilion and 72.2 per cent table, 39 degree pavilion and 58.4 per cent table.
Diamonds with these proportions show fish-eyes that require no tilt to see them. If the table gets 1 per cent bigger one sees a 1 per cent more fish-eye.
We down grade fish-eyes in value, a bit like an inclusion, because that is what they look like.
A small amount of tilt to see a fish-eye is acceptable because these diamonds have a very good spread and look very big for the money. If the fish-eye can only be seen with five degrees or more tilt, then the diamond is considered to be ideal.
Spread is very important, but hardly ever talked about by technicians working in diamond grading labs. The purchaser wants their diamond to look big. The diamond on the left of the photograph “sounds” big but looks smaller than the one on the right.
Both measure 5.2 mm in diameter and could have been cut from the rough diamond in the centre. Note the lifeless one is 3.7mm deep while the ideal cut is just 3.1mm.
How does one know if the diamond one is considering has a good spread? Divide the depth by the diameter and multiply by 100 to calculate the depth percentage.
This should be between 56 per cent and 65 per cent. If the table is small, the depth will need to be larger, and vice-versa. The smaller the depth percentage the larger the spread. Other factors that affect spread are the girdle thickness and crown and pavilion angles.
The girdle is the edge of the diamond and this is measured in relative thickness and the type of finish.
Girdles used to be bruted, which meant two diamonds were ground round on each other. This resulted in a dull, waxy appearance. These days most larger diamonds are faceted with lots of very small flat facets or they are polished smooth. A faceted girdle does not improve a diamond’s grade, although a bruted girdle looks much worse if the stone is a fish-eye.
Girdle thickness should ideally be between thin, medium and slightly thick. There is almost always some variation in girdle thickness around a stone, often there are small four thin areas reflecting the original shape of the octahedral rough diamond crystal. These thin areas should not be set in exposed positions because these are often cleavage directions. If a diamond is struck in these directions extremely thin to thin girdles can and will chip.
Diamonds with thick, very thick, or extremely thick girdles weigh considerably more but are still worth buying if the price is right. There may be very small amounts of light loss when viewed in some directions.
While very thin girdles are not recommended for claw set rings, they are fine in pendants and earrings. Even set this way there is always a risk a jeweller will chip the girdle while setting the diamond if it is too thin.
The point on the bottom of a diamond's pavilion is called a culet (pronounced que-let or que-lay). During the manufacturing process, the culet is often polished as a flat facet so that it does not get chipped as the other facets are polished. Ideally, the cutter “closes” this facet to a point, but sometimes it remains as a small extra facet.
A diamond behaves as a window if opposing facets are parallel, and this is exactly what happens if the culet is too big. One can see straight out the hole in the bottom. The culet size is listed on a cert and the diamond's culet should be pointed (no culet), very small, small or medium because these are not visible to the naked eye. Large to extremely large culets may be visible to the naked eye and can be treated almost as though they are inclusions.
Symmetry is graded as Excellent, Very Good, Good, Fair or Poor on a GIA report. When polishing a rough diamond, the aim is to cut the heaviest, most valuable diamond possible. This often means polishing a diamond with imperfect symmetry to avoid inclusions or just leaving more weight to achieve a "magic weight" (like 1.00-carat). The polished diamond may be slightly off round, have variations in girdle thickness, tilting of the table, and off centring the table or the culet, etc.
Often symmetry defects in a diamond are the result of great skill, rather than an indication of poor skills.
The diamond images used in this tutorial are mostly symmetrical. But in the real world, very few diamonds are perfectly symmetrical. Symmetry is less important to the overall beauty of a diamond than the critical facet proportions. One may never notice any difference between diamonds with excellent or ideal and very good or good symmetry.
Polish is graded the same way as symmetry by most labs: Excellent, Very Good, Good, Fair or Poor. Just as hardwood takes a better polish than softer timbers, a diamond’s hardness makes it the absolute leader in lustre. Poorly polished facets may reduce the intensity of light reflected from, or refracted into and out of, a diamond. Diamond grading labs assess polish by examining the diamond, facet by facet, with reflected light with a microscope.
A common polish defect is surface grain lines. As they polish each facet, even the most skilled cutter can encounter variations in hardness or grain, just like with timber. The result is very fine polishing lines running across a facet.
These grain lines are very common in pink and fancy-coloured diamonds, but are rarely visible to the naked eye. If one chooses a diamond with SI or VS inclusions, a few microscopic polish lines may be of no relevance. But for those considering buying a flawless diamond, excellent polish may be a consideration.
If the polish is rated as fair or poor, visual performance may be noticeably reduced, or one may be able to see a polish line on the crown or the diamond.
About a third of diamonds fluoresce, like the fluorescent minerals seen in natural history museums, or the novelty shop toys under the black (UV) light. The effect is like a white shirt in a nightclub.
Fluoro can be faint to very strong, and the most common fluorescent colour is blue. As blue is the complimentary colour to yellow, the most common tinted colour in diamonds, blue fluorescence can make yellowish diamonds look white or colourless.
A GIA survey found that fluorescent diamonds were favoured over non-fluoro stones, especially in lower colours, but even in the higher colours (D, E and F) which are often discounted by the trade. Many years ago, colourless fluorescent diamonds were highly-prized and referred to as "blue-white".
But salespeople used the term too loosely for any diamond with fluorescence; "blue-white" usage was outlawed by US trade practices laws.
One "for" argument for discounting fluorescent diamonds is because the GIA lab grading lights emit a small amount of ultra violet light; fluorescent diamonds might be assigned a better colour grade. The “against” argument, although the GIA Gem Trade Lab has not openly discussed the issue, is that UV light is almost always present in viewing environments, so why not grade colour in realistic lighting?
But the most likely reason for fluoro diamond discounting is because of the sad fact that many jewellery salespeople are not able to explain complex phenomena like Fluoro; a Fluoro (or any comments) written on a report makes the diamond harder to sell and thus, worth less!
Imagine this sales-killing explanation: "Fluorescence is visible light emitted by electrons when a diamond is excited by higher energy sources (Ultra Violet light or X-rays)."
Some diamonds have extremely strong fluorescence and appear oily or cloudy. This is bad. But the GIA study found them to so rare that they were unable to find enough cloudy stones from the 26,010 samples to conduct a study of them. It is advised that one not to buy a Very Strong unless one can actually see the diamond side by side with non-fluorescent diamonds in shaded daylight (which has a lot of UV light).
This author’s experience from the sales floor confirms the GIA findings: most people would choose a fluorescent diamond over a non-stone anyway. The fact it may cost less is a real bonus. Rarely diamonds fluoresce another color like yellow or orange. Do not buy them unless the diamond concerned is a fancy colour of the same hue as the fluorescence (which will make it more intense). White diamonds with yellow or orange fluoro will appear to be a lower colour when seen in light with a UV component. When the UV light is turned off fluorescence ceases instantly, but some stones continue to phosphoresce for a little while.
Posted August 26, 2009