Moonstone
Moonstone isn’t a separate mineral species — it’s a variety of orthoclase feldspar (or, in some material, the closely related feldspar adularia), and what earns it a name of its own is an optical effect called adularescence: a soft, floating sheen, usually blue-white, that seems to move beneath the surface as the stone is tilted. The effect comes from thin, alternating internal layers of two feldspar types that formed together as the crystal cooled slowly, scattering light the way oil on water scatters color. It’s a structural phenomenon, not a coating or treatment, which is why it survives cutting and polishing but is lost entirely if a stone is cut the wrong way relative to its internal layering.
Color body ranges from colorless through white to pale blue-grey, and the best “blue moonstone” — prized for a strong, cool-toned sheen against a nearly transparent body — commands significantly more than the milkier white material seen in mass-market jewelry. Sri Lanka has long been the source most associated with fine blue moonstone; India, Myanmar, Madagascar, and the United States also produce gem-quality material.
At Mohs 6 to 6.5, moonstone sits in the moderate range — harder than opal or turquoise, softer than quartz or topaz. It also has feldspar’s characteristic cleavage planes, meaning a sharp knock in the wrong direction can chip or split a stone even though the surface itself resists scratching reasonably well. Rings see more impact than earrings or pendants, so moonstone jewelry worn daily benefits from a protective setting and a bit of care around rough activity; ultrasonic cleaners are best avoided given the cleavage risk, with a soft cloth and mild soap the safer routine.
Moonstone’s birthstone standing is solidly modern. It appears as a June stone — usually paired with pearl — on the US 1912 list, the UK’s 2013 list, and the current US list (which adds alexandrite to the same month). The older historical list, by contrast, lists it only as a secondary August stone alongside sardonyx, carnelian, and topaz, with June instead assigned to cat’s eye, turquoise, and agate. That’s a useful illustration of how much birthstone months moved around before the trade associations standardized them in the 20th century. Moonstone does not appear in either zodiac birthstone system tracked on this site — its identity is tied to the monthly lists, not the astrological ones.
Gemmological properties
| Mohs hardness | 6–6.5 |
|---|---|
| Crystal system | Monoclinic |
| Chemical formula | KAlSi3O8 |
| Refractive index | 1.518–1.526 |
| Color family | White, Colourless, Blue |
| Variety of | orthoclase feldspar |
| Also known as | — |
| Origins | Sri Lanka, India, Myanmar, Madagascar, United States |
| Organic material | No |
Moonstone is a variety of the feldspar mineral orthoclase (or the closely related adularia), prized for adularescence — a floating blue-white sheen caused by light scattering off internal feldspar layers.
Where Moonstone appears as a birthstone
By month
Frequently asked questions
- What causes moonstone's glowing sheen?
- Adularescence — a floating, billowy blue-white light caused by internal layers of two feldspar minerals (orthoclase and albite) scattering light as it passes through the stone. It's a structural effect, not a coating, and it's strongest in stones cut as smooth domed cabochons rather than faceted.
- Is moonstone the same as adularia?
- Closely related but not identical in trade usage. Adularia is a specific, especially clear form of orthoclase feldspar historically mined in the Adula Alps; 'moonstone' is the broader gem-trade name for feldspar showing adularescence, whether or not it's true adularia.
- Which birthstone lists include moonstone?
- It's a June stone on the US 1912, UK 2013, and current US lists, and a secondary August stone on the older historical list — but it does not appear on either zodiac birthstone system tracked here.