Tanzanite is a more recently discovered gemstone that can only be found in one place: the hills of Merelani in the country of Tanzania, after which it is named. Its value depends mostly on its color, though its cut, clarity, and weight also influence its valuation.
High color saturation in the stone is most prized. While Tanzanites are pleochroic, meaning their color can vary in different light, the most valued is a deep blue similar to sapphire, followed by a shade of violet-blue. While popular, gems that carry both purple and blue within its body are less valuable, and pale colors decrease in value significantly. Interestingly, in nature, tanzanite appears brown. The color you see in finished, cut stones is a result of heat treatment and cutting.
Valued for clarity and color saturation, any inclusion in the gem decreases its value. An inclusion is when material trapped in the gem during formation is visible on the gem's surface.
Since tanzanite is pleochroic, the way it is cut can drastically affect its value. Gems of purple-blue color have generally been cut less than gems that are a bright blue, a choice by cutters to conserve the weight of the gem while cutting. This is why vibrant blue gems hold more value; they are less-commonly produced and require more work to create.
Blue stones have higher value per carat than other variations of tanzanite. The finest colors and higher-valued stones usually weigh 5+ carats, while less vibrant tanzanite tends to appear in smaller sizes.
Safe for normal wear, it should not be exposed to sudden changes in temperature, which means no ultrasonic or steam cleaning. Instead clean with warm, soapy water and soft cloth.
Tanzanite was discovered in 1967, so it's fairly new to the gem market. Discovered by Merelani locals, it quickly gained popularity, which lead Tiffany & Co. to sign an exclusive deal as the main proprietor of the gemstone.