A "mixed cut" gemstone is typically defined by a step-cut pavilion and brilliant crown, and you'll often see these in many traditionally cut gemstones. "Radiant" cuts are the opposite - usually a brilliant style pavilion and step cuts on the crown. It's a fairly new cut and works well to deepen the saturation in light colored stones. Once this characteristic became known to diamond cutters, many light yellow diamonds in the GIA W-X diamond color range were recut with a radiant design because it increased their apparent color saturation, thus giving them a more desirable Fancy Yellow color. In the sapphire example you can see how the radiant cut concentrates and deepens the pink color in otherwise light pink material.
I've received a lot of questions lately about the various types of lab-created gemstones, and which type is "best". There are many creation types but for the three most common are flame-fusion, hydrothermal, and Czochralski pulled (there are others but these are rarely available to cutters). Flame-fusion is the oldest and least expensive, and is typically used for ruby, sapphire and spinel. Gemstone size is limited to approximately 10 carats, and some colors are difficult to replicate well due to color zoning (blue sapphire is a good examples). Hydrothermal is primarily used for beryl (emerald, aquamarine, morganite, etc) since it mimics their natural growth processes, but gemstone size is limited due to the thin shape of the rough. The hydrothermal technique can also be used for ruby and sapphire, but it's relatively uncommon since other methods offer lower costs, better colors and larger sizes. The Czochralski pulled technique is the primary technique for YAG and alexandrite, but it can be used for ruby and sapphire. The primary advantages of this type are that the color is uniform throughout and that very large gemstones can be cut.
In summary: Hydrothermal: Expensive, best for emeralds and other types of beryl. Limited color range and size.
Czochralski Pulled: Expensive, best for YAG, alexandrite, blue sapphire and/or extremely large stones. Limited color range.
Flame fusion: Inexpensive, available in a wide range of colors, best for ruby, spinel and most colors of sapphire (except for blue - see above).
Sapphires often have color zoning (which typically is undesirable) but this feature can be used to create cool effects when incorporated into the design. Here's an example where the blue color zoning in the culet was used to highlight the "flower" in this lab-created sapphire.
Finewater Gems is thrilled to announce its third place award in the Phenomenal Category of the American Gem Trade Association Spectrum Awards for this flawless 45.59 ct Ethiopian crystal opal. The design and cutting was performed over a period of several months to maximize color play, long-term stability, and beauty. The AGTA Spectrum Awards serve as the benchmark for industry design excellence and are the most important design competitions in the world for colored gemstones and cultured pearls. See the press release.
Unlike diamonds, there are no clarity standards for colored stones. "Eye-clean" is an informal definition used by many in the trade, but it is often misinterpreted by customers. Unlike most other vendors, Unlike most other vendors, we provide highly detailed photos that give a view similar to the use of a 10x loupe, so you may see inclusions in our photos that you would never notice in real life. With a bit of effort you may notice several tiny inclusions in this photo of a VS clarity amethyst. It is not flawless (nor is it claimed to be), but it is unlikely that anyone could see these inclusions at normal viewing distances without magnification - hence the description "eye-clean".
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.