Tanzanite is a beautiful gem. In addition to its beauty, it has some properties that require it to be given special care. Tanzanite is best suited for earrings, pendants, and other jewelry items that will not encounter abrasion and impact. It is less suited for use in a ring. Many jewelers recommend that “tanzanite rings are for dress rather than daily wear.”
Hardness is the resistance of a gem to being scratched. Tanzanite has a hardness of about 6.5 on the Mohs Hardness Scale. This hardness is low enough that the gem is vulnerable to being scratched during normal wear if it is used in a ring. This problem can be reduced if the setting is designed to protect the stone from impact and abrasion, or if the ring is not worn during activities when the chance of impact or abrasion is high.
Toughness is the resistance of a gem to breakage. Tanzanite has one direction of perfect cleavage that could result in the gem being chipped or broken if it receives a sharp impact. Again, settings can be designed to protect the stone, and the owner can avoid wearing the piece during certain activities to reduce the probability of damage. Tanzanite is also sensitive to sudden temperature change and is more subject to breakage at that time.
Color stability in tanzanite is good. Heat-treated stones hold their color and are unlikely to fade under the normal light exposure and in the temperature range of the human environment. As with all gems, contact with acids and other chemicals should be avoided because the stone might be etched or damaged. If cleaning is needed, warm water and a mild soap are recommended. Steam and ultrasonic cleaning are not recommended.
| Physical Properties of Tanzanite | |
| Mineralogy | Tanzanite is a blue color-variety of the mineral zoisite. |
| Color | Blue to bluish purple to bluish violet. The color of commercial stones is almost always produced or enhanced by heating brown or green zoisite. Strong pleochroism: dichroism and trichroism. Tanzanite is seen in a wide range of color saturations and tones that make it an alternative stone for aquamarine, blue topaz, and sapphire. |
| Luster | Vitreous |
| Diaphaneity | Transparent. Often of high clarity. |
| Cleavage / Fracture | Perfect cleavage in one direction. |
| Mohs Hardness | 6.5 |
| Specific Gravity | 3.35 |
| Diagnostic Properties | Defined by its blue color, usually with a distinct purple or violet tint. Strong pleochroism, specific gravity, hardness, refractive index. |
| Chemical Composition | Ca2Al3(SiO4)3(OH) |
| Crystal System | Orthorhombic |
| Uses | Gemstone |