Tanzanite is a relatively modern gemstone that has rapidly gained popularity due to its captivating violet-blue coloration. This unique gem is a variety of the mineral zoisite, and it is known for its strong pleochroism, which means it can display different colors when viewed from various angles. The gemstone’s exceptional nature is defined by its single known source in the world, a small area within the Merelani Hills of Tanzania, which is also the origin of its name. The captivating beauty of tanzanite comes with specific considerations regarding its physical properties, particularly its hardness and overall durability in jewelry.
Defining Tanzanite’s Hardness
The hardness of a gemstone is measured on the Mohs scale, which assesses a mineral’s resistance to scratching. This qualitative scale ranges from 1, representing the softest mineral (talc), to 10, the hardest mineral (diamond). Tanzanite falls within a hardness range of 6.5 to 7 on this scale, which places it in a moderate position among colored gemstones. The Mohs scale is not linear, meaning the jump in hardness between two consecutive numbers is not equal across the entire scale. For instance, a diamond at 10 is vastly harder than a sapphire at 9. This specific 6.5 to 7 rating for tanzanite indicates its relative susceptibility to abrasion when compared to harder materials.
Practical Implications of Its Hardness
The 6.5 to 7 hardness rating has direct practical implications for how tanzanite jewelry should be worn and cared for. Quartz, which rates a 7 on the Mohs scale, is one of the most common materials found in everyday environments, especially as airborne dust. Since a material can be scratched by any substance with an equal or higher hardness rating, tanzanite can be scratched by common dust particles. This means tanzanite is less suitable for high-impact jewelry like a daily-wear ring compared to stones with higher hardness ratings. For comparison, a sapphire or ruby scores a 9, and a diamond tops the scale at 10. While tanzanite is hard enough for use in most jewelry, its moderate rating requires mindful wearing to preserve its polished surface and brilliance.
Beyond Hardness: Understanding Tanzanite’s Durability
While hardness refers strictly to scratch resistance, a gemstone’s overall durability depends on two other factors: toughness and cleavage. Toughness is the resistance to breaking, chipping, or fracturing, and tanzanite is generally classified as having fair to poor toughness. This inherent characteristic means the stone is relatively brittle and can fracture easily upon impact. A significant structural detail is tanzanite’s perfect cleavage in one direction, which is a plane of weakness within the crystal structure. Cleavage means that a sharp blow delivered in a specific direction along this plane can cause the crystal to split easily, requiring careful handling during setting and wearing.
Safe Handling and Storage
Given its moderate hardness and susceptibility to cleavage, tanzanite requires specific precautions in handling and cleaning. Avoid using ultrasonic cleaners or steam cleaners, as the high-frequency vibrations and heat can exploit the gem’s internal cleavage planes and cause the stone to fracture. Instead, the safest cleaning method involves warm, soapy water and a soft brush. Tanzanite is generally stable under normal wearing conditions, but it should not be exposed to sudden temperature changes or harsh chemicals like hydrochloric or hydrofluoric acids. Proper storage is also important; tanzanite jewelry should always be kept separately in a soft pouch or fabric-lined box to prevent contact with harder gems, such as diamond or sapphire, that could easily scratch its surface.