The Formation of Moonstone ~ The Shimmering Gemstone of June

The Geological Recipe: How Moonstone Forms

The formation of moonstone begins deep within the Earth's crust, under high pressure and temperature conditions. Here's a simplified step-by-step process:

1. Cooling of Molten Rock (Magma)

Moonstone starts its journey as part of igneous rock, which forms when molten rock (called magma) cools and solidifies. During this cooling process, minerals begin to crystallize. If the cooling happens slowly enough — often over millions of years — crystals have time to form properly.

2. Intergrowth of Two Feldspar Types

As the magma continues to cool, two different types of feldspar — orthoclase (a potassium-rich feldspar) and albite (a sodium-rich feldspar) — begin to grow together. At first, they mix together as a single, uniform mineral.

But as the rock cools further, something fascinating happens: the two feldspar types start to separate into stacked, alternating layers. This process is called exsolution, which means the minerals “unmix” themselves over time. Think of it like oil and water separating out after being shaken together.

3. Formation of Thin Layers

The albite and orthoclase create incredibly thin layers, often only a few hundred nanometers thick (about 1,000 times thinner than a human hair). These stacked layers are responsible for the shimmering effect of adularescence.

When light enters the moonstone, it scatters between these thin layers. The light bends (or diffracts) as it hits the layers, bouncing back and creating the glowing sheen we see.

How Long Does it Take?

The creation of moonstone is not a quick process. It happens over tens of millions of years, as part of the geological life cycle of the Earth.

1. Formation of magma and slow cooling – millions of years.

2. Crystallization and separation of feldspars – also takes millions of years, depending on the conditions deep within the crust.

3. Erosion and exposure – after the moonstone forms underground, it may take additional millions of years for natural processes like erosion or volcanic activity to bring it to the surface where it can be mined.

So, the moonstone in your June jewelry might have begun its formation before the dinosaurs went extinct!

Moonstone Crystals

Why Are There Different Colors?

While the most commonly recognized moonstone is a pearly white with a blue glow, moonstone can actually come in a variety of colors. The cause of these color variations depends on several factors:

Chemical Composition

Slight differences in the proportions of potassium and sodium in the feldspars can change the way light interacts with the layers. For example:

• A higher albite content might produce a bluer shimmer.

• More orthoclase can give a warmer, golden sheen.

Crystal Structure and Layer Thickness

The thickness of the albite-orthoclase layers affects the wavelength of light that is scattered:

• Thinner layers scatter blue light, producing that classic bluish glow.

• Thicker layers reflect warmer tones, like yellow, orange, or even green.

Inclusions and Impurities

Other minerals or tiny fractures inside the stone can influence its color. For example:

• Iron content might lead to a more orange or reddish tone.

• Microscopic inclusions (tiny particles trapped inside the crystal) may create interesting light effects, such as a “cat’s eye” or a glittery look.

Location of Formation

Different mining locations yield different types of moonstone. For example:

• Sri Lanka is famous for high-quality blue-flash moonstone.

• India produces moonstones with a rainbow-like effect, sometimes called “rainbow moonstone,” although technically it's a different type of feldspar called labradorite.

• Myanmar (Burma) and Tanzania are also notable sources.

Moonstone Stones