Tanzanite is a mineral, defined as a naturally occurring solid substance with a definite chemical composition and a regular, repeating crystalline structure. It is a specific variety of the mineral species zoisite, found in the Earth’s crust. The intense blue-violet stone was a relatively recent discovery, first found in the late 1960s, quickly establishing itself in the jewelry world.
Classification and Chemical Makeup
Tanzanite is classified as the blue-to-violet variety of zoisite, which is a calcium aluminum silicate. The mineral has a consistent chemical formula, written as Ca2Al3(SiO4)(Si2O7)O(OH). The rich color that makes tanzanite so popular is caused by trace amounts of the element vanadium substituting for aluminum within the crystal lattice.
The mineral belongs to the epidote group and is categorized as a sorosilicate, which is a specific arrangement of silicate tetrahedra. This internal structure dictates the external shape of the crystal, which forms in the orthorhombic crystal system. This system results in prismatic crystals that often display distinct striations on their surfaces.
Geological Origin and Exclusive Source
The formation of tanzanite is tied to highly specific geological processes that occurred approximately 585 million years ago. It formed under intense heat and pressure conditions, estimated at around 650 degrees Celsius and pressures of 5 to 6 kilobars, during regional metamorphism. This metamorphic activity transformed pre-existing rocks, specifically graphite-bearing gneisses and schists, near the base of Mount Kilimanjaro.
The mineral precipitated from hydrothermal fluids rich in elements like calcium, aluminum, and the color-causing vanadium. These fluids migrated into fractures and tension sites within the host rock. The crystallization of tanzanite occurred in these openings, often alongside other minerals like grossular garnet and tremolite.
The exclusivity of tanzanite comes from the unique convergence of these specific conditions in one location: the Merelani Hills of the Simanjiro District in Tanzania. All known commercial deposits of gem-quality tanzanite are contained within a very small mining area, roughly 7 kilometers long and 2 kilometers wide. This single-source origin makes tanzanite one of the rarest gemstones on Earth.
Distinct Optical and Physical Characteristics
The appearance of tanzanite is characterized by a remarkable color change that is responsible for its high value as a gemstone. When first mined, the crystals often exhibit a reddish-brown or yellowish-brown hue, which is not the famous blue-violet color. The vibrant color is achieved through a common heat treatment process, where the stone is heated to temperatures between approximately 370 and 480 degrees Celsius.
This heating process alters the valence state of the vanadium ions, removing the undesirable brownish component and maximizing the stone’s saturation of blue and violet. The most striking optical property of tanzanite is its strong pleochroism, which is the ability to display different colors when viewed along different crystallographic axes. Untreated tanzanite is trichroic, showing three distinct colors: blue, violet, and a reddish-brown or yellow-green.
After heat treatment, the stone typically becomes dichroic, showcasing the two most desirable colors, blue and violet, with varying degrees of intensity. Jewelers must carefully orient the rough crystal during cutting to ensure the finished gem displays the most appealing face-up color, often prioritizing the blue or violet axis. These optical features are paired with physical properties that affect the stone’s durability for use in jewelry.
Tanzanite measures between 6.5 and 7 on the Mohs scale of hardness, making it softer than many other popular gemstones like sapphire. This relative softness means the stone is susceptible to scratching and abrasion, requiring protective settings when worn in rings. The mineral possesses perfect cleavage in one direction, meaning it has a distinct plane of structural weakness where it can split if struck sharply. For this reason, tanzanite is often recommended for less exposed jewelry items, such as earrings and pendants, to minimize the risk of damage.