Generally, it is recommended that the gemstone you choose for an engagement ring measures at least 7.5 on the Mohs hardness scale. Tanzanite, however, tends to measure between 6 and 7, meaning that it is not as tough as other gemstones that are used in engagement rings.
It is a stone that is not particularly tough, and one that is also prone to cleavage: if struck, tanzanite stones can break. In an engagement ring, which is subject to daily wear and tear, this isn't ideal.
However, with the right setting that protects the stone, the risk of damage can be minimised and tanzanite rings can last a lifetime.