Which of the following gemstones is known for forming in hydrothermal conditions?

Emerald, ruby, and aquamarine are all gemstones that can form in hydrothermal conditions, making "all of the above" the correct answer. Hydrothermal processes involve the movement of mineral-rich water through rock, where changes in temperature, pressure, and chemical environments promote the crystallization of minerals.

Emerald typically forms in metamorphic rocks, often through hydrothermal activity that introduces beryllium, vanadium, and chromium into its structure. Rubies, which are red corundum, primarily develop in metamorphic rocks but can also crystallize in hydrothermal veins. Aquamarine, a variety of beryl, forms similarly under hydrothermal conditions, often in granitic pegmatites where water serves as a solvent to transport essential elements like beryllium and aluminum.

This background illustrates the significance of hydrothermal processes in the formation of these gemstones, and thus the assertion that all the listed options are known for forming in hydrothermal conditions is accurate.