Clarity of natural gemstones refers to the volume of external flaws or internal flaws (inclusions) in each stone. Such internal formations are present in almost all gemstones. That is why finding a flawless gemstone is rarer than finding a flawless diamond. During the formation process of natural gemstones, several processes occur within the earth. These processes determine the number of inclusions a stone will have in the end. Some gemstones like emerald form under extreme conditions. Thus, such stones will always have internal formations. Darker gemstones like the purple amethyst, blue topaz and red garnet include more flaws. Yet, that does not lessen the value of the stone because the depth of their colour conceals the flaws. While lighter gemstones show internal inclusions better. The most obvious example is diamond. Sapphire, on the other hand, does not usually exhibit the high clarity of a fine diamond. Sapphires include several inclusions, and even the best sapphires are not free of flaws when a cutter views them at 10x magnification. Also, sapphire inclusions differ according to their sources and treatment history.
Depending on the clarity, there are different types of coloured gemstones. The classifications reflect the differences in the geological conditions forming the stones; and how those conditions affect the appearance of the gem (with and without magnification). For grading clarity, the GIA proposes three main categories of coloured gemstones - Type I, Type II, and Type III.
Type I - Gemstones in which inclusions are not noticeable. They form under geological conditions that are not very severe. Such gemstones are often "eye-clean".
Type II - Gemstones with the presence of noticeable inclusions. They form under more severe conditions. Sapphires are obvious examples.
Type III - Gemstones with inclusions visible to the unaided eye. They form under extreme geological conditions. These stones are seldom "eye-clean". Cut
Coloured gems are available in different shapes and cuts that enhance their natural beauty. The best cuts intensify and reflect light evenly without any windowing or darkness in the stone. There exist no rigid set of rules for cutting coloured gemstones. Most often, it is the cutter that makes a choice depending on the inclusions present in a gemstone. The cutter works around the best option around such flaws to minimize their impact and maximize the beauty of the stone.
A gemstone comes with multiple facets. While the facets on the top of the stone (crown) have the function of capturing light, the facets on the bottom (pavilion) reflect light internally. This capturing and reflection of light produces different visual effects. Unlike for a diamond, a cutter uses mixed styles while cutting coloured stones. The popular cuts include a coated pavilion with a concave-cut dome or a scissor-cut crown with a step-cut pavilion.
To enhance the appearance of a gemstone and to give it a reflective finish, it is often polished. This process allows light to refract through the stone and reflect off the surface. Different types of objects can help polish a gemstone. Some of those include fine diamond grates and metal oxides like ferric or aluminium oxide. Other notable options include leather, wood and felt. The polish of a gemstone makes a significant impact on its sparkle.