Dispersion ("fire") is the characteristic of gemstones to take in white light and return spectral rainbow colors to the viewer's eyes as the gemstone or light source moves. The effect is dependent upon the material as well as the cutting design (high crowns can generate more dispersion, like the one shown in this photo). While most colored gemstones have modest amounts of dispersion, well-cut light violet or gray spinels often show more dispersion than should be possible.
"Hearts and Arrows" designs are a gold standard for many diamond buyers, but are very uncommon in the colored stone world. What's the attraction? It's not so much the pattern that can be seen with special viewers or photographs, but rather that it's an easy way to quickly identify which gemstones have excellent proportions, symmetry, cutting, clarity, and polish. All of these need to be absolutely perfect in order to generate this pattern, which results in a brilliant and dazzling gemstone. If you see this or a similar pattern in a gemstone it's proof that it's been cut "just right".
Spessartite garnets were named after the first discovery in the German Spessart Mountains, and have since been found in a number of locations - mainly in Africa. Often called "Mandarin" garnet because only the high ranking officials in ancient China (the mandarines) were allowed to wear orange. More durable and brilliant than both orange citrine or tourmaline, and substantially less expensive than orange sapphire, spessartite garnets are a great choice for an orange gemstone.
Very light stones can be the most difficult to photograph as they tend to reflect too much light back to the camera, masking the delicate details and dispersion seen in zircons or diamonds. This is a good time for "less is more" - that is, less light can make a better photo - as in this lovely 3.19 ct white zircon.
Inclusions in gemstones are generally not a desirable feature; however when the inclusions are very small and evenly distributed the light is even scattered and extinction reduced, as in the case of this unheated and "glowy" blue sapphire.
These beautiful sapphires are the rarest and most desirable of all sapphires, and the original (and some say the only true variety) were from Sri Lanka. The color is similar to the lotus flower so the name in the Sinhalese language are padma (lotus) and raga (color), or padmaraga, modified over time to padparadscha. Some have also likened it to the color of the sunset or saffron. The combination of ruby and yellow sapphire produces a lovely orangey-pink to pinkish-orange color that is the critical defining characteristic, with no historical mention of tone or saturation. For unknown reasons an influential gemologist in the 1980’s decreed that they must have a pastel tone and low saturation, and this notion has stuck with Western gem labs and consumers. Fortunately the rest of the world still includes all sapphires in the color range as padparadschas, regardless of tone or saturation. I’ve been purchasing padparadschas in Sri Lanka for years, and I much prefer the rare and richly saturated medium toned stones.
If you need a special gemstone, just ask! As a GIA trained graduate gemologist with extensive travel experience and industry affiliations, I have excellent contacts in source countries such as Sri Lanka, Burma, Thailand and Colombia. Contact me for more details.
Some new and unusual golden grossular garnets were available at the Tucson gem show this year, like this large 14.26 ct hessonite garnet. Hessonite is a subcategory of grossular, so how do gemologists tell a hessonite garnet from a normal grossular garnet? We observe the stone under magnification, where a hazy "heat wave" effect is considered diagnostic for hessonite.
When viewing gemstones, a neutral gray background reduces color bias and allows your eyes to see true and accurate color. This is why GIA recommends that a gem's color be evaluated against a neutral background, why the Photoshop application uses a gray background, why illustrators like their working areas to be gray, and why a neutral gray card is used as my studio photography backdrop. Black, white or other colors can make dramatic photo backgrounds, but anything other than neutral gray can easily result in a less-than-accurate color representation of a gemstone.