What gem is formed by metamorphism?

The correct answer is sapphire, which is a variety of corundum that forms under conditions of metamorphism. Metamorphism occurs when minerals are subjected to high temperature and pressure, leading to changes in their physical and chemical properties. Sapphires can arise from metamorphic rock environments where aluminum and other elements combine under these intense conditions, resulting in the characteristic crystalline structure that defines this gem.

In contrast, opal is a hydrated silica that typically forms through the precipitation of silica from water; it is not a product of metamorphic processes. Tanzanite, while a valuable gemstone, is formed from the mineral zoisite, which originates from metamorphic processes but is more commonly associated with thermal metamorphism rather than the ideal conditions required for sapphire formation. Quartz, although it can form in metamorphic rocks, is not classified specifically as a metamorphic gem and can also crystallize from igneous and sedimentary processes. Thus, sapphire stands out as the definitive gem that forms as a result of metamorphism.