Moonstone: 5 Types Told Apart by Sheen and Body Colour

Hold a fine moonstone against a window and rotate it slowly. Its body colour may be colourless, white or tinted. A soft blue-white sheen can appear to move across the polished surface as the viewing angle changes. The effect comes from light reflected by very fine feldspar intergrowths inside the stone. Geologists call the effect adularescence.

Moonstone appears under several trade names. “Rainbow moonstone” usually refers to labradorite. “Blue moonstone” describes the colour of a sheen, while “cat’s eye moonstone” refers to a narrow, directional light band. The names describe different aspects of feldspar materials.

1. Blue moonstone. Near-colourless body, strong floating blue sheen. Classic alkali-feldspar moonstone; Sri Lanka is a well-known source.
2. White moonstone. Milky translucent body, soft white sheen. Usually the same alkali feldspar, and a common bracelet material.
3. Rainbow moonstone. Colourless body with multicoloured flash. This is usually labradorite, a different feldspar, sold under the moonstone name.
4. Peach moonstone. Warm apricot body colour with sheen that varies between specimens. A colour name used for feldspar gem material.
5. Cat’s eye moonstone. A single bright band that moves across a dome. Chatoyancy, a different optical effect from adularescence.

What moonstone is

Moonstone is a gemmological name for feldspar material with a characteristic moving sheen. Feldspars are among the most abundant mineral groups in the Earth’s crust. The term adularia denotes a low-temperature potassium feldspar (ideal formula KAlSi₃O₈) named for the Adula Alps in Switzerland. It is not a synonym for moonstone; typical adularia lacks the exsolution texture responsible for classic moonstone sheen. Hardness is 6–6.5 on the Mohs scale, harder than glass and softer than quartz, with two near-perfect cleavages that intersect at almost 90°.

The optical effect comes from a structural detail. Classic moonstone has fine intergrowths of orthoclase (potassium feldspar) and albite (sodium feldspar), arranged as alternating lamellae on a sub-micron scale. The two phases unmix from a single high-temperature feldspar as the crystal cools, in a process called exsolution. The thickness of each lamella is comparable to the wavelength of visible light, which is what makes the optical mechanism interesting.

When light reaches the boundaries between fine orthoclase and albite regions, some of it is reflected. Reflections from the internal layers interfere, creating a sheen that changes with the viewing angle. Different intergrowth textures can produce blue, white or other iridescent colours. The apparent colour alone does not identify the feldspar.

Adularescence, labradorescence, chatoyancy: separating the optics

Four optical terms appear in discussions of moonstone. Their mechanisms and appearances help describe the sheen in a polished bead.

Where moonstone forms

Classic alkali-feldspar moonstone develops its sheen when a feldspar crystallises first and its orthoclase and albite later unmix into very fine, regular lamellae as it cools, in rock that leaves those lamellae intact. The cooling history varies by deposit, and the strength and direction of the sheen are evaluated on the finished bead.

Sri Lanka is a documented historical source of blue-sheen moonstone. Rainbow moonstone material is also recorded from India and Madagascar. Colour, clarity and the movement of the sheen describe the appearance of each stone.

Reading a moonstone strand

Moonstone quality is judged above all by light behaviour, together with body colour, clarity and cut. Hold a strand against a daylight bulb, in motion.

For another light effect in finished jewellery, compare moonstone and natural opal.

• Direction of the sheen. Adularescence travels in a particular direction relative to the crystal’s internal structure. In a well-cut bead, the cutter has aligned that direction with the bead’s axis, so the blue moves smoothly as the strand rotates. In a poorly cut bead, the sheen sits in a fixed patch and disappears as you turn the wrist.
• Depth of the sheen. The reflection may appear close to the surface or deeper within a bead, depending on internal texture, transparency and viewing angle.
• Body transparency. The finest moonstone is nearly colourless under direct light, with the blue only emerging on rotation. A grey or cloudy body reduces the optical contrast; flashy “rainbow” material accepts a more opaque body because the colour effect is stronger.
• Cleavage planes. Look for fine straight lines crossing the bead at near right angles. These follow the two cleavage directions of feldspar. They are normal, and on their own they do not prove the material.
• Bead-to-bead consistency. Compare the strength and direction of the sheen across the strand. Notice how brighter and softer reflections relate from bead to bead.

Trade names, decoded

Several trade names describe feldspar materials by their colour, optical effect or historical source.

• Rainbow moonstone. A widely accepted trade name usually used for transparent labradorite, a calcium-sodium plagioclase. Its iridescence may show several colours, while classic moonstone often has a softer blue-white sheen. Their appearances can overlap, so the flash alone is not a mineral identification.
• Ceylon moonstone. A historical geographic trade name referring to Sri Lanka, formerly called Ceylon.
• Peristerite. A sodium-rich plagioclase whose fine intergrowth gives a blue sheen. It is mineralogically distinct from classic moonstone and is sometimes sold under the moonstone name.
• Cat’s eye moonstone. Genuine moonstone showing chatoyancy from oriented inclusions. The eye effect is a separate phenomenon from adularescence; the best stones show both.
• Star moonstone. Rare four-rayed asterism in moonstone.

Peach, Grey, Black and Green Moonstone

Peach and pink. These describe warm body colours, including orange-yellow to peach shades. The body colour and the moving sheen are separate observations.

Grey and black. Moonstone body colour can range from grey to nearly black. “Black moonstone” is a commercial colour label, without a fixed mineral formula.

Green. Green body-coloured moonstone is documented, including a southern Indian example reported by GIA. A green body colour differs from a green iridescent flash in rainbow moonstone.

Peristerite and related feldspar trade names

Peristerite is a name for iridescent sodium-rich plagioclase with fine exsolution intergrowths. Like labradorite, it belongs to the plagioclase feldspars; classic moonstone has a different alkali-feldspar intergrowth. Their blue sheen can sometimes appear similar, which helps explain the overlapping trade names.

Caring for a moonstone strand

Moonstone has a Mohs hardness of about 6 to 6.5 and pronounced cleavage. Handle a finished strand gently, wipe it with a soft cloth and store it separately from harder jewellery. Avoid ultrasonic and steam cleaning. More detail is available in crystal jewellery care.

How BE. grades and selects moonstone

BE. applies a four-axis system, BE. Crystal 4T™, to every strand: Transparency, Tone, Texture, Treasure. For moonstone, Texture reads the sheen, its depth, direction and movement, together with bead polish and the absence of surface haze; Tone reads the body colour and how even it is along the strand; Transparency reads how clearly the body transmits light around the sheen; and Treasure records the source country. BE.'s moonstone is from India and untreated. Each strand ships with a Stone Origin Record listing the lot number, the source country and the treatment.

Further reading

GIA and Mindat provide descriptions of moonstone and feldspar materials. Ribbe's Feldspar Mineralogy and Webster's Gems cover additional mineralogical and gemmological detail. Related topics include natural materials and imitations and crystal jewellery care.

References

• Mindat: Moonstone (variety of feldspar)
• Mindat: Orthoclase
• Mindat: Labradorite
• GIA: Moonstone
• Wikipedia: Moonstone (gemstone)
• Ribbe, P.H. (1983). “The chemistry, structure and nomenclature of feldspars.” Reviews in Mineralogy, 2: Feldspar Mineralogy.
• Webster, R. (2002). “Gems: Their Sources, Descriptions and Identification”, 5th ed. Butterworth-Heinemann.