GemTopic of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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June, 2004

DISPERSION

Dispersion, refers to an optical property of gemstones whereby flashes and pinpoints of spectral colors are displayed as the stone is turned in the light. The dispersive colors we see are not really there in the gem, instead, they are created by the behavior of white light in the stone. Dispersion results when light passes out of a transparent material with inclined surfaces (like a prism, or a faceted gemstone). Although the term "fire" is gemologically equivalent to dispersion, "fire" is so frequently misused to mean either brilliance (total light return) or scintillation (twinkling), that I prefer to use "dispersion" in all my descriptions for the sake of clarity.

White light is, of course, made up of a spectrum of wavelengths from relatively long (red) to relatively short (blue and violet). Each of these wavelengths is bent to a different degree (red less, blue more) when passing from air into a denser medium like a gemstone. When the bent light waves exit through an inclined surface (like a facet), depending on the degree of bend (or refraction), they may show as distinct spectral colors. The ability of a gem species to show dispersion is, therefore, roughly correlated with the density and refractive index of the gem material itself.

This property is a distinctive characteristic of each gem species and can be used in the process of identifying a gem. Testing for dispersion in gems, however, is actually a rather painstaking and complicated process involving measuring the separate refractive indices of the red and blue wavelengths in that species and calculating their ratio. Instead, most gemologists and gem lovers simply gauge dispersion by eye.

There are published tables (see below) of the laboratory values for each species, but in actuality several other factors may enhance or depress the display in a given stone. Foremost among these is body color. For example, two species with high values for dispersion: demantoid garnet and Benitoite tend to have fairly dark body color which usually masks the effect to a great degree. Fans of dispersion and fans of rich color often part company over which is more beautiful --- a saturated medium dark blue Benitoite which shows little of its potential dispersion, or a substantially lighter one with spectral colors flashing at every turn. (As a card carrying Benitoite fancier, I am definitely in the latter category!) The 2.0 ct. Benitoite stone in this pendant has to my eye an optimal balance between color and dispersion.

 

The two sphalerites below show the effect of darker body color on dispersion, the yellow stone showing much more than the darker orange stone.

[Yellow Sphalerite / Orange Sphalerite]

In general, the larger, cleaner and lighter in color the gem, the more of its potential dispersion will be visible. Cutting style has a noticeable effect as well, in that higher crowns accentuate the effect and flatter crowns diminish it. (I once cut a large, light lavender spinel (a species with modest dispersion figures) with an extremely tall crown that had more "fire" than a lot of poorly cut diamonds I have seen.

Diamonds have always been admired for this property, and so diamond simulants have been sought that have similar dispersive characteristics. Up until a few decades ago, the diamond simulant of choice was white zircon, whose dispersion and high luster make it a good visual replica for diamond.

[Diamond / White Zircon]

Synthetic rutile and strontium titanite were each briefly popular when first synthesized, but were noticeably more dispersive than diamond and so, not very convincing.

[Synthetic Rutile / Strontium Titanite]

YAG (yttrium aluminum garnet), was also used, but with dispersion noticeably less than diamond, it was unsatisfactory as well.

[YAG]

Cubic zirconia now has the lion's share of the simulant market, among other reasons, because its dispersion, though higher than diamond is close enough to look right, especially in sizes most commonly used in jewelry.

[Cubic Zirconia, CZ]

In the list below are a few species that might be seen in today's marketplace that typify gems with low, moderate, high and very high dispersion values:

Low

Moderate

High

Very High

 

Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment

-- Mail to barbara@acstones.com

{Search our Catalog}