The best of the blues: hue and tone

Blue sapphire, like ruby, is a primary color gemstone. The purer the primary hue the better. In practice this means that a dark-toned (seventy to eighty percent) primary blue hue with no more than a ten to fifteen percent secondary purplish hue is most desirable.137 Some connoisseurs prefer a distinct purplish secondary hue because it adds a velvety richness to the blue; others prefer a purer, more “open” blue of slightly lighter (seventy-five percent) tone. This range of hues is to be considered the finest color in sapphire.

Green is the bane of blue sapphire. Any visible hint of green brings a stone’s value crashing down into the commercial range. The problem is that all blue sapphire has a greenish component when viewed at certain angles to the C axis. It is the cutter’s job to cut the stone so that this green is not part of the face-up appearance. Often this slight tint of green is difficult to see. Stone-to-stone comparison will often highlight the green secondary hue, as long as the viewer is not looking at a series of slightly greenish stones.

Saturation

Gray is the normal saturation modifier in blue sapphire. Often it will be found mixed with green in lower-quality stones. A slight gray mask will introduce a cool or slightly “steely” quality in the normally warm hue of a sapphire. All pure chromatic hues are vivid. If the key color appears dull and cool, a gray mask is the probable culprit.

Multicolor effect 

Since blue sapphire is a primary color gem, the closer it comes to exhibiting a uniformly pure primary blue hue the better and more desirable it is. The face-up mosaic of a gemstone, however, is far from uniform; each facet may exhibit variations in the gem’s key color. Some facets may appear bright, some dull; some may display a dark tone, others a medium- or light-toned blue.

Multicolor effect has several causes (see Chapter 4). Sapphire is dichroic, i.e., light entering the gem divides into two rays, one violetish blue, one greenish blue. In addition, the stone may be zoned: colorless zones are juxtaposed against zones of color. Light rays passing through colorless zones lose color. Also, the pavilion facets of the gem cause a light ray entering the stone to reflect at least twice within the stone, absorbing color as it goes. Add to this the effect of light that shifts in color temperature from yellowish to bluish, accentuating or depressing the purplish secondary hue, and you have an idea why the face-up scene may be less
than uniform.

In ruby and sapphire a negative type of multicolor effect is traditionally called bleeding, manifested as a lightening of tone and a loss of saturation when the stone is shifted from natural to incandescent lighting. At lighter tones, blue becomes pastel, less saturated, and washed out. Bleeding is a good analogy: the color is drained from the stone just as blood is drained from the body. The effect is similar: the stone becomes pallid, and its life, figuratively, is drawn out of it. Bleeding in sapphire may be described as weak, moderate, or strong. The more apparent or stronger it is, the greater the fault.

Kashmir sapphire contains little or no chromium, which appears to be one cause of the multicolor effect. What this means is that Kashmir sapphire, unlike Burma sapphire, for example, will not pick up a purplish secondary hue, but will maintain its hue as the viewing environment is shifted from daylight to incandescent lighting.

Crystal

With gemstones there is one truth: no matter how fine the stone, somewhere there is a better one. Sapphire at its zenith — that is, a stone that seems to have everything: color (hue, saturation, and tone), clarity, and marvelous make — still requires a velvet transparency to reach the very pinnacle of quality.138 Blue sapphire often will close up in incandescent lighting.

This may have little effect on color, so it can’t be described as bleeding; the crystal simply becomes turbid, dark, and murky as the lighting environment is shifted from natural light to the light of the bulb.

Some stones will have the three Cs (color, clarity, and cut), but very few also have the diaphaneity, the good crystal — the fourth C, imparting a quality that is rich, crisp, and velvety all at the same moment. It is this quality that finally separates the very finest from the rest of the herd. Fine stones that fit this description may come from any source. Beauty is its own best pedigree.

Texture

Color in sapphire often occurs in zones that follow the hexagonal outline of the sapphire crystal. Zones of rich color will alternate with colorless areas. This is particularly characteristic of gems from Sri Lanka. Due to zoning, sapphire will often show what experts call texture. This means that the zones are sometimes visible face up, causing the color to appear uneven. Even color is very important in sapphire; it is the cutter’s job to integrate the zones so that the face-up color appears to be even. What is seen through the side or back of the stone is of little importance.

Sometimes the lapidary’s attempt to even out the texture by eliminating the effect of the natural zoning in the crystal leads to poor symmetry.

However, stones that appear lopsided below the girdle or that have off-center culets are more tolerated in sapphire than in most other gem species. Symmetry faults that would be considered major flaws in diamond are accepted in sapphire so long as they occur below the girdle and do not create a lopsided girdle outline.

Heat enhancement

No one is quite sure how long heat-enhanced sapphire has been in the market — perhaps a hundred years, probably much longer.139

Heat treatment, known as burning, was reported in India as early as 2000 bc.140 However, it was not until the 1970s that the technology to achieve very high temperatures became available, and heat treating began to be practiced on a grand scale. Some lighter Ceylon stones (thirty to fifty percent tones) are unheated. But most of the finer Ceylon and Tunduru stones, as well as a good portion of the Madagascar stones currently in the market, are heat enhanced.

Heating has a negative effect in sapphire. Heat-treated blues have generally poorer crystal than unheated stones. The heating process tends to reduce transparency or muddy the crystal. 141 If all other factors are equal, the very best natural color sapphire will be more beautiful than the very best burned sapphire. For example, of the top ten blue sapphires in the world, two through nine may be heat enhanced, but number one will be natural color. The exact opposite occurs when ruby is subjected to heat treatment. In general, natural color sapphire will sell at a premium of approximately thirty percent above the price of a comparable heat-enhanced stone.

With regard to gemstones in general, all factors are rarely equal. Sapphire, like all other gems, should be considered stone by stone. Heat treatment improves the appearance (hue, saturation, tone, clarity) of a vast majority of sapphires; otherwise, it would not be done.

The rarity factor

Exceptional blue sapphire is rare in any size. Stones over twenty carats are available: however, as with most gem species, stones larger than those readily usable in jewelry on a per carat basis tend to decrease in price.