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Sep 19, 2002
What is it on the HCA?

Garry H (Cut Nut)

Aug 15, 2000
18,559 has the pictures
Here is the copy:

Brilliance, Fire, Scintillation and Spread

Brilliance is the human perception of diamond brightness.

Brilliance is not simply light return, it involves complex issues that include scintillation or contrast with the added variable of human perception. Never-the-less a diamond with poor light return cannot display optimal beauty.

Poor light return can be as a result of a number of factors

leakage, like a window effect that allows a diamond to be seen through, which results in a dead appearance when set in jewelry. This leakage is easily detected using an Ideal-Scope.

shallow and very deep diamonds can have poor light return because of the ‘Harding effect’ discussed in section 5 on previous research.

Diamonds in the lower left of the table of diamond images in the review of the GIA brilliance study show excessive darkness because of the observers head blocking light supply.

Fire is the term used to describe flashes of color resulting from spectral separation or dispersion of white light into rainbow flashes.

Maximum brilliance and fire cannot both be observed in a single view at the same point on a diamond, a brilliant white flash will flood out any colored light as seen in the example here. (Figure 8).

When you view a diamond in a flood lit office style room, it appears very bright, whereas in a point lit room the same diamond will appear darker but more firey.

Two factors are taken into account when assessing fire for HCA. The first was the well known relationship between steeper crown angles and fire. The second is a discovery I made during this study – it involves the amount of darkness in a diamond in the face-up position;this darkness enhances the visibility of fire. Some HCA scores for the fire of very shallow diamonds will be increased slightly to account for this effect.

Figure 11: These two views of an identical virtual Tolkowsky diamond are as a one eyed observer would see them from 60cm (left) and 20cm. In the second stone the observer’s head blocking illumination enables our eyes to see fire that is overpowered by an abundance of white light in the first stone, which has less light sources obscured by the viewers head.

Another discovery I made concerns darkness that results from light leakage. Paradoxically leakage inside the table of the most commonly proportioned diamonds (as shown in the Ideal-Scope comparison in the Brilliance section) allows reverse light leakage entering via the pavilion, to be displayed as fire emerging out this leakage area of the table. The trade find these stones acceptable (even desirable) because when held in tweezers and viewed and lit the way dealers do, they appear very attractive. But once set they die.

Steep / deep diamonds should be set so light can get in the pavilion. They should also be kept as clean as possible.

Scintillation is often thought of as contrasting dark and brightness that alternates from facet to facet and attracts our eyes as the diamond, the illumination or the observer moves. The GIA authors defined scintillation as “flashes of light reflected from the crown”. I don’t like this definition at all; reflected light is part of overall light return. Observer rarely seperate reflected from refracted light.

I will define two types of scintillation.

Static Scintillation is the amount and the placement of darkness in a diamond seen in a face up view in a jewellery store environment from a distance of 16 inches (400cm).
Dynamic scintillation is the sparkle effect you see when a diamond, the light or the observer moves.

The later is very difficult to define. HCA is based on static face up scintillation.

Scintillation is so personal that defining or measuring it may well be impossible. What scintillation does is provide contrast for brilliant white, or firey colored sparkles, making them appear brighter to the human eye. Presumably as part of our inherent survival instincts, we are able to detect the tiniest movement in our field of view. We are attracted to sparkly things.

It is common to refer to diamonds with additional facets as having more scintillating, like the Brilliant Rose™, which has an additional set of facets on the pavilion of a traditional round brilliant. However some consumers prefer the bolder blocky scintillation of old miner cuts. Eye sight is obviously an important factor.

As noted above, a facet cannot simultaneously display a burst of white light and a burst of fire, fire is enhanced when a flash of color emerges from a facet darkened by a shading of a light source (or a dark zone caused by light leakage). A further refinement of this concept concerns the proximity of a dark facet to a flash of white or colour. Consider the appeal of a black opal, which appears more striking than an equally colourful white opal.

Diamonds with poor symmetry can display poorly distributed scintillation.

Spread or apparent size of a diamond is the easiest factor to understand, and to many diamond buying consumers, it is the most important ‘desirability’ factor. The spread has been ignored in previous cut grading systems, but HCA compares and rates it.

A diamond that looks bigger than another is not necessarily more beautiful, but it is more desirable. A diamond of the same weight but larger diameter is said to ‘spread’ more. Spread is a matter of desirability rather than a beauty factor.

First time consumers often mistakenly consider the spread of a diamond to be directly related to its weight. Within the trade, the depth percentage is used as a value factor in determining spread, but depth percentage is a poor metric when used alone because weight is affected more by girdles and crowns than pavilions.

For many consumers everything about the beauty of a diamond is salesmanship, because after all even ugly diamonds are beautiful; to some size is all that counts, and having advice about the size rather than the weight is very important.
The scores from zero to 2 were given using a similar comparison method as used for brilliance and fire. You can clearly see the pleasing star patterns in the chart in the GIA brilliance review section.

Unlike fire and brilliance (the more, the better) the ideal amount of scintillation is in the eye of the beholder; the observer is an important variable. A person with excellent eyesight may examine diamonds from a distance of 8 inches (20cm) whereas a viewer with poor eyesight may hold a diamond at arms length to focus on it (common with advancing age). A person with poor eyesight may find a Tolkowsky proportioned stone has insufficient contrast1. She may find a diamond with shallower crown and pavilion angles and shorter pavilion half facets more scintillating.

The GIA’s ‘Modeling Brilliance’ article chose to use an observer free illumination and images and photographs displayed in Figure 2 show the effect of no scintillation.

Another variable in considerations of scintillation is observer appearance. A bald fair skinned person in white clothing viewing a diamond will observe less scintillation than a dark person wearing a big black hat and a black suit. The latter will see more darkness in diamonds.

The bow tie in a marquise is the ‘nail head’ or observer reflection (or that of a camera or microscope lens). A nail head results when pavilion angle is greater than 43.5° causing a table-to-table reflection of the viewers own head (Harding 1975). A view of the same marquise (or a nail head) through a peephole in a sheet of white paper magically turns the bow tie white.
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