Which synthetic gem is produced solely by the hydrothermal growth process?

The correct choice indicates that quartz is produced solely by the hydrothermal growth process. Hydrothermal growth is a method used to create synthetic gemstones by mimicking the natural geological processes that form crystals deep within the earth.

For quartz, this process involves creating conditions similar to those found in hydrothermal veins—hot, mineral-rich waters that deposit crystals as they cool. The resulting synthetic quartz can often be indistinguishable from natural quartz, and the hydrothermal method is particularly suited for this type of crystal because it allows for the controlled growth of high-quality specimens.

The other options—emerald, diamond, and sapphire—can be created through various synthetic methods, not exclusively hydrothermal growth. For instance, synthetic emeralds are typically made using the hydrothermal method as well, but they can also be produced using the flux growth method. Synthetic diamonds are usually created through high-pressure, high-temperature (HPHT) processes or chemical vapor deposition (CVD), while synthetic sapphires are commonly produced using both the Verneuil process and hydrothermal methods. Thus, quartz stands out in this context as the gem that is produced solely by hydrothermal growth.