What gem is formed by metamorphism?

Sapphire is the correct answer because it is a type of corundum that forms through metamorphic processes. During metamorphism, pre-existing minerals undergo changes in temperature and pressure, which can result in the transformation of aluminum and oxygen into corundum, creating sapphires. This process often occurs in regional metamorphic environments where high pressure and temperature facilitate the recrystallization of minerals.

In contrast, opal is a mineraloid formed from silica gel and is not the result of metamorphism. Tanzanite, while sometimes associated with metamorphic processes, primarily forms from the thermal metamorphism of zoisite, which does not classify it as a pure metamorphic gem in the traditional sense. Quartz, on the other hand, is commonly found in a wide range of geological settings, but it typically crystallizes from magma or sediment rather than through metamorphism. Thus, sapphire stands out as the gem that is distinctly formed through the metamorphic process.