Most people think of garnets as a red stone with a slightly brownish cast. You may be surprised to know that garnets come in every color except blue. The two kinds of green garnets called tsavorite and demantoid are rare and consequently more expensive than red garnets. Garnets are hard, durable and brilliant. Garnets are valued on carat weight, color, cut and clarity. The value is based on the factors as a whole. Two garnets of the same weight could have very different values because the color of one is inferior to the other. Or one could have poor clarity or an inferior cut.
Weigh the garnet. Weight is measured in carats and is a major factor in value. Usually the bigger a garnet looks the more it weighs. However, the cut of the gemstone can make it look bigger. For example: a rectangular cut that is broad and shallow looks larger than a round cut that is narrower and deeper.
Determine the color. The purer the color and the closer it is to the spectral red, the more valuable the stone. Spectral red is the red seen when a shaft of light is broken down into colors. A rainbow is a good example of spectral colors. Most red garnets have that hint of brown or look more orange than red. The exceptions are the green garnets because they are so rare. A fine green garnet rivals an emerald for beauty and could easily be mistaken for one by someone unfamiliar with gemstones.
Look at the distribution and depth of the color within the stone. A lighter red is less valuable than a deeper red. Some gemstones show streaks or bands of lighter or darker colors when viewed from the top or the sides. Consistent color throughout the stone is more valuable.
Appraise the quality of the garnet's cut. Cut refers to the quality of how the stone is cut, not the shape it is cut in. Garnets are faceted so the light entering the stone is reflected and bounces around in the garnet showing off the stone's brilliance. If the facets are not lined up correctly, the light leaks out of the stone and some of that brilliance is lost.
Examine the garnet under a jeweler's loupe of 10 times (10X) magnification. Most stones including garnets have small inclusions, cracks, fissures or specs of other material within them. If the flaws can not be seen under the 10X magnification, the stone is considered flawless. Garnets with visible flaws are not as valuable as stones without flaws.
Consider the total value of all the factors. For example a 4.95ct (weight) VS (clarity grade) tsavorite garnet (rare green color ) is valued at about $1,700, while a garnet nearly the same weight 3.79ct (weight) VS-SI (clarity grade) spessartite garnet (red color) is valued at $200 as of 2010, according to Gemselect.com.