Moonstone

Moonstone is a sodium potassium aluminium silicate of the alkali feldspar group that displays a pearly and opalescent schiller (adularescence). This optical phenomenon is caused by light diffraction within a microstructure consisting of regular exsolution layers of different alkali feldspars (orthoclase and albite). Ancient Romans believed that moonstone was formed from solidified rays of the moon.

Formation & Geology

Moonstone forms in plutonic and volcanic rocks. The characteristic sheen is produced during formation when the orthoclase and albite separate into alternating layers as the mineral cools. When light falls between these thin, flat layers, it scatters in many directions producing the phenomenon called adularescence.

Physical Characteristics

Moonstone has a hardness of 6 to 6.5, making it somewhat soft compared to quartz or corundum. It has two directions of perfect cleavage, which makes it prone to breaking if struck hard. The most valuable moonstones are colorless with a blue sheen.

Historical Significance

Moonstone has been used in jewelry for millennia, including ancient civilizations. The Romans admired it, believing it was born from solidified rays of the moon. Both the Romans and Greeks associated moonstone with their lunar deities. It became popular again during the Art Nouveau period.