Tanzanite Hardness & Durability

Tanzanite Oval Tanzanite Hardness and Durability

Tanzanite is the crystalline form of the mineral zoisite and has a Mohs hardness of 6.5 to 7.0. At first glance this may not seem to be a very hard gemstone when compared to sapphire at a 9.0, diamond at a 10.0, chrysoberyl at 8.5 and the beryls at 8.0. See the chart below.

Due to its cubic crystalline structure, diamond is very hard, but it is also very brittle, due to its plane of cleavage, and would be prone to fracture more easily than, let's say, jade. Jade is much softer than diamond, but has a very "tough" monoclinic crystalline structure and can withstand a very good knock while sustaining a small amount of damage.

Hardness, as the concept relates specifically to gems and minerals, is primarily referring to scratch resistance with secondary considerations given to cutting resistance. Referring to the above chart, one can see that tanzanite, having a hardness 6.5 to 7.0, is only slightly less scratch resistant than quartz, which is what amethyst and citrine are. Considered quite durable, amethyst, with a trigonal crystalline structure, is only slightly more scratch resistant than tanzanite, with a orthohombic crystalline structure. Please see our section on pleochroism and definitions for an explanation of terms.

You may be asking why the hardness of tanzanite is expressed as a range of numbers. The answer is that tanzanite crystals have 3 axes (pl. axis) of differing lengths. The shortest axis will be the hardest and the long axis will possess the lower number.

Actually, 6.5 to 7.0 hardness is quite durable, though care should be exercised to make sure that tanzanite is not knocked or bumped sharply. Ultrasonic cleaning of tanzanite is discouraged. Over time, almost all gemstones, with the exception of diamond possibly, will develop some type of wear on them. Tanzanite is simply "middle-of-the-pack" in terms of hardness. Simply use a little care and common sense when wearing your tanzanites and they should provide you with many years of sparkling beauty.

Tanzanite Hardness ~ Discussion

"Scratch Hardness" was a concept developed by Friedrich Mohs who lived in the late 18th to the 19th century. He used 10 different minerals with differing hardnesses to develop his 1 to 10 scale, with 1 being lowest and 10 being highest, or most resistent to scratching. He discovered that minerals with the same hardness would also scratch each other. The scale is used widely today and is known as, "Mohs' Hardness Scale". The table below shows the comparative and absolute hardness values to help you understand this concept more completely.

Absolute and Relative Gemstone Hardness Chart
Mohs' Hardness
Mineral Used Simple Test Tool
Cut Resistance
Talc Fingernail
Gypsum Fingernail
Calcite Copper Coin
Fluorite Knife (easily)
Apatite Knife
Orthoclase Steel File
6.5 to 7.0
Quartz Will scratch glass


Tanzanite Pear Tanzanite Trilliant Tanzanite Cushion Tanzanite Oval

If you should have any questions or concerns regarding the care of your tanzanite, please contact Customer Service through the email link provided on the "Contact Us" page.

Tanzanite Oval 4.91ct