Which gemstone is included among those that often form in pegmatites?

• A. Moonstone
• B. Emerald
• C. Opal
• D. Lapis Lazuli

Answer: (A)

Pegmatites are late-stage, coarse-grained pockets in igneous rocks that concentrate feldspar and other minerals, allowing very large crystal growth. Moonstone is a gem variety of the feldspar group, and its formation is common in pegmatites where these big feldspar crystals develop and can exhibit the adularescence that gives moonstone its distinctive glow. Opal forms from silica-rich solutions in volcanic or sedimentary settings, not typically pegmatites. Lapis lazuli comes from a limestone-rich environment with minerals like lazurite and is not a hallmark pegmatitic gemstone. Emerald, a beryl, can occur in pegmatites but is more famously linked to metamorphic and hydrothermal settings. So moonstone is the gemstone among these most closely associated with pegmatites.