In natural gemstones, clarity refers to the amount of internal flaws (inclusions) or external flaws of each stone. Most gemstones have some amount of inclusions, and finding a flawless gemstone is rarer than finding a flawless diamond. When natural gemstones are formed, the process within the earth that creates them is what determines the number of flaws a cut stone will have. Some gemstones, like the emerald, are formed under violent circumstances, and, therefore, are almost always included. Darker gemstones-like the blue topaz, purple amethyst, and red garnet-can carry more inclusions without lessening the value because the depth of their color masks most flaws. Lighter gemstones will show internal inclusions much more clearly. The diamond is the most obvious example, although it is less common to find an internally flawless emerald than an internally flawless diamond. Sapphires rarely exhibit the high clarity of fine diamonds. Sapphires host many different inclusions, and even the best sapphires are not expected to be free of inclusions when viewed at 10x magnification. In fact, sapphire inclusions vary with their source or origin and treatment history.
Where clarity is concerned, colored gemstone are divided into several classifications reflecting differences in the geological conditions under which they are formed and how those conditions affect the physical appearance of the gemstone, with and without magnification. Colored gemstones are basically divided into three categories by GIA for purposes grading clarity; Type I, Type II and Type III.
Type I – Gemstone form under geological conditions in which inclusions are not very noticeable and such gemstone are often "eye-clean."
Type II – Gemstone form under geological conditions that are more severe, resulting in the presence of inclusions that are typically more noticeable. Sapphires fall into Type II gemstone.
Type III – Gemstone form under the most violent geological conditions, resulting in gemstone that typically show inclusions, often to the unaided eye, and are rarely "eye-clean."
Colored gemstones come in all sorts of shapes and cuts that are designed to enhance their natural beauty; the best cuts enhance and reflect light in an even manner without any darkness or windowing in the stone. There are no fixed rules for cutting colored gemstones, and often the cutter makes a choice based on the inclusions in the gemstone in order to work best around any such flaws so as to minimize their impact.
The facets on the top or crown of the gem have the function of capturing light, while the facets on the bottom or pavilion reflect light internally. This capturing and reflecting light produces several different visual effects.
Gemstones are polished to give a reflective finish, in order to allow light to refract through the stone and reflect off the surface. A variety of materials can be used to polish a gemstone, including very fine diamond grades, metal oxides, like aluminum oxide or ferric oxide, or felt, leather or wood. A gemstone’s polish is largely connected to its sparkle.