Which gem is known to crystallize in volcanic rock due to gases released by magma?

The gem known to crystallize in volcanic rock due to gases released by magma is indeed red beryl. Red beryl forms in conditions where volcanic activity leads to the release of gases, creating the right environment for its development. This unique formation process often occurs in rhyolitic lava flows, where the high heat and pressure create the ideal conditions for the growth of this rare gemstone.

In contrast, other listed options such as topaz, amethyst, and peridot typically crystallize in different geological conditions. Topaz primarily forms in igneous rocks, particularly granites and pegmatites but does not specifically crystallize from volcanic gases. Amethyst, a variety of quartz, often originates in volcanic and sedimentary environments as well, but not directly from volcanic gases. Peridot, which is an olivine mineral, usually crystallizes in the Earth's mantle and is associated with basaltic volcanic rocks but is not specifically formed due to gas emissions in the same manner as red beryl.