Moonstone is a captivating gemstone, a form of the mineral feldspar recognized for its distinctive optical effect called adularescence. This phenomenon gives the stone its characteristic shimmering, milky light that appears to float just beneath its surface. Chemically, the gem is a potassium aluminum silicate. Its origins are geographically diverse, linking its availability to a global network of specialized mining operations.
The Geological Context of Moonstone Formation
Moonstone belongs to the feldspar mineral group, which makes up more than half of the Earth’s rocky crust. The classic moonstone is typically an orthoclase feldspar, though other feldspars like the plagioclase variety can also exhibit the moon-like sheen. The adularescence effect occurs because the stone’s internal structure is composed of alternating, microscopic layers of two different feldspars—orthoclase and albite. These layers separate during the cooling process through a mechanism called exsolution. Moonstones form within igneous rocks like granite and pegmatite, where slow cooling allows large crystals to develop, or occasionally within metamorphic settings.
Primary Global Sources of Classic Moonstone
The most highly valued moonstone, known for its clear body and electric-blue adularescence, has historically come from a few select locations. Sri Lanka, formerly Ceylon, is considered the traditional source of the finest blue moonstone, particularly from the village of Meetiyagoda in the southwest. Mining in Sri Lanka is typically done using artisanal, small-scale methods, where miners dig shafts by hand to reach the moonstone-bearing gravel deposits. Historically, Myanmar (Burma) was also a producer of exceptional, highly transparent moonstones with a strong blue sheen, though this source is now largely depleted. Tanzania has also emerged as a producer, yielding stones that display an attractive, unique schiller effect.
Sources of Specialized Moonstone Varieties
While Sri Lanka and Myanmar are known for classic blue and white stones, other regions supply the market with various colors and specialized types. India is a major source, producing moonstones in a spectrum of hues, including brown, orange, and gray. India is also the leading producer of the variety marketed as “Rainbow Moonstone.” This material is actually a colorless plagioclase feldspar known as labradorite, which displays a multicolored sheen called labradorescence. Madagascar offers a range of moonstone colors, including peach and gray, and has become an important commercial source, alongside limited deposits found in the United States.