As with other gems, the value per carat of tanzanite is influenced by color, cut and clarity. Other factors jewelers use to grade tanzanite include shape and polish.
The price per carat of tanzanite can vary from a few dollars to thousands based on the clarity, color and cut of the stone. Inclusions, or interior impurities, and blemishes, or exterior impurities, affect the value of individual stones. If a gemstone is clean, meaning there are no visible inclusions or blemishes, then it is valued higher than a stone with either or both. However, if the inclusion does not impede the brilliance of the tanzanite, then it doesn't decrease the value by much.
Stones that display a higher saturation of color are valued higher than those with less depth. A deep blue tone with good depth in the gemstone is rare, making it a valuable characteristic of tanzanite.
Cut is another major factor in grading stones for price. The goal of cutting stones is to maximize their luminescence, usually via facets. When a skilled jeweler has cut a quality stone with many well-placed and well-cut facets, it displays maximum brilliance. These stones garner a higher price.
Along with the cut of the stone goes its shape and polish. A stone should be well-polished with no scratches. Round stones tend to be more expensive, as more raw material is lost during the cutting process.