Which colored stone variety faces cutting challenges because of its vulnerable cleavage planes?

The correct answer is Tanzanite, which poses cutting challenges due to its distinctive cleavage planes. Tanzanite, a variety of the mineral zoisite, exhibits strong pleochroism and is prized for its vivid blue and violet hues. However, it also has perfect cleavage in one direction, making it susceptible to splitting or breaking if not handled with care during the cutting process. This characteristic can complicate the gem cutter's task, as they must orient the stone carefully to avoid damaging it while maximizing its visual appeal.

In contrast, while the other colored stones listed do have their own properties that affect cutting and handling, they do not exhibit the same level of vulnerability due to cleavage. Rubellite and Topaz are generally tougher, and while Emerald does have its challenges related to fissures and inclusions, it is not renowned for having vulnerable cleavage planes like Tanzanite. This makes Tanzanite particularly challenging for gem cutters who must be adept in their techniques to produce high-quality finished stones.