MOONSTONE

Gemological Information on Moonstone (Adularia)

On this occasion I want to delve into one of the gems that generates the most confusion even among professionals: Moonstone, a commercial name used for both Adularia and White Labradorite —also known as Rainbow Moonstone. As a result of this mixture of terms, many jewelers and buyers encounter different materials under the same name, which is why it is essential to clarify the differences from a serious and updated gemological approach.

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Interesting facts about Moonstone

The denomination “Moonstone” has changed in meaning over time. Adularia is the classic Moonstone, historically described for its gentle adularescence. However, in recent decades White Labradorite has flooded the market, becoming the most common variety, with a more intense and colorful optical effect.

As described in reference materials, today it is even more difficult to find authentic adularia, while white labradorite has become the predominant variety due to its past abundance and greater visual appeal to most buyers.

Gemological aspects of Moonstone

Although the name “Moonstone” encompasses two materials, their gemological parameters allow them to be differentiated:

Adularia (Potassium feldspar)

• Chemical composition: KAlSi₃O₈

• Crystal system: monoclinic

• Hardness: 6 – 6.5

• Optical effect: gentle, milky, or silvery adularescence

• Transparency: translucent to semi-transparent

White Labradorite (Plagioclase feldspar)

• Chemical composition: (Ca,Na)(Al,Si)₄O₈

• Crystal system: triclinic

• Hardness: 6 – 6.5

• Optical effect: multicolor labradorescence (blues, pinks, violets)

• Transparency: translucent

Origin

• Adularia: formerly more common in Sri Lanka, although today it appears infrequently in the market.

• White Labradorite: extracted in significant quantities in Madagascar, India, and formerly Sri Lanka.

According to the reference document, two decades ago there was considerable abundance of white labradorite in Sri Lanka, which led to it being commercialized under the name Rainbow Moonstone, displacing adularia.

Imitations and treatments

Common imitations

Moonstone is one of the most counterfeited gems:

• Synthetic: laboratory-created adularia exists, with lighter weight and intense adularescence.

• Glass or “opaline”: artificial material sold massively as Moonstone, with plastic appearance.

• Other minerals: selenite, scapolite, and some quartzes are easily confused with adularia depending on the cut.

The document highlights that opaline —an opaline glass— is sold worldwide as Moonstone, despite not being a mineral.

Treatments

Adularia is usually not treated, as it does not withstand thermal or pressure processes.
White labradorite is also usually not treated, but can be selected based on the intensity of the optical effect.

How to identify them

• Adularia presents gentle and uniform adularescence.

• Rainbow moonstone shows more intense multicolor flashes.

• Avoid completely homogeneous materials or those with plastic sheen.

Geological formation

Both varieties belong to the feldspar group, forming in environments:

• Magmatic: crystallization in lavas and pegmatites.

• Metamorphic: recrystallization of feldspathic materials.

The internal layered structure is responsible for the characteristic optical effect of both varieties.

Uses in jewelry

Moonstone is used mainly in:

• Cabochons for rings, pendants, and earrings.

• Artisanal pieces where the interplay of light is enhanced.

• Contemporary designs that seek to highlight adularescence or labradorescence.

Its moderate hardness allows relatively safe use, although it is recommended to avoid impacts and settings that are too exposed.

Value of Moonstone

The factors that determine its value are:

• Optical effect: intensity, distribution, and color of the phenomenon.

• Transparency: the greater it is, the more valuable, especially in adularia.

• Base color: soft milky or slightly bluish tones.

• Provenance and rarity: authentic adularia has higher value due to its low availability.

White labradorite is well-positioned in jewelry for its striking aesthetics, while adularia is increasingly scarce and sought after by collectors and designers who seek a more classic material.

How to evaluate quality

To differentiate qualities:

• Verify the uniformity of the optical effect.

• Avoid excesses of opaque zones or surface fractures.

• Identify whether it is adularia or labradorite, as their value is different.

• Confirm it is not opaline glass or another imitation.

Consult more gemological criteria here:
https://samsaragemstones.com/informacion-gemologica/que-es-la-gemologia-2/

Conclusion

Moonstone is a gem that combines optical beauty and jewelry tradition, but it is also a material that requires knowledge to avoid confusion. Both its classic variant (adularia) and the popular rainbow moonstone (white labradorite) offer exceptional aesthetic possibilities, provided that naturally sourced and correctly identified specimens are selected.