Moonstone

A member of the feldspar group.

Feldspars form complex isomorphous series between the endmembers albite and anortite on the one hand and the endmembers albite and orthoclase on the other hand. Members of the albite-anortite series are summed up under the term plagioclase. Members of the albite-orthoclase series are termend alkali feldspars.

Classical moonstones are orthoclase feldspars.

Moonstone shop


mondstein moonstone sri lankamondstein moonstone sri lankamondstein moonstone sri lanka

Blue moonstones from Sri Lanka

Origin of Name: from the bluish silvery sheen that reminds of moonlight.

The effect is also called "adularescence", named after a feldspar location in the Swiss Adula mountain range.

The name orthoclase derives from ancient Greek ὀρθός (orthos) = upright, perpendicular and κλάσις (klasis) = to break and hints to the angle of the cleavage planes.

Synonyms and trade names: none. There is, however, a number of other feldspars, in particular labradorite (albite-anorthite series), being sold as moonstone. Multicoloured labradorite (spectrolite) is often sold as "rainbow moonstone".

spectrolitespectrolitespectrolite

"Rainbow moonstone" (spectrolite) from Bihar, India

In German speaking countries "adular" is sometimes used synonymously to moonstone, although adular actually is an orthoclase variety with a typical crystal habit

Can be confused with: blue moonstone can be confused with blue labradorite.

labradorite

Blue labradorite from Tanzania
 

.blauer mondstein blue moonstone ceylon sri lankablauer mondstein blue moonstone ceylon sri lankablauer mondstein blue moonstone ceylon sri lanka

Three fine blue moonstones from Sri Lanka

Sri Lankan moonstones, or Ceylon moonstones, as they are still called in the gem trade, sometimes feature interesting Illmenite inclusions that look like a rope-ladder through loupe or microscope

Ceylon moonstone wih Illmenite inclusions

Quantitatively the mooonstone deposits of South India are most important.

mondstein katzenauge moonstone cat´s eye indien indiamondstein katzenauge moonstone cat´s eye indien indiamondstein katzenauge moonstone cat´s eye indien india

mondstein grau grey moonstone indien indiamondstein grau grey moonstone indien indiamondstein grau grey moonstone indien india

mondstein schwarz black moonstone indien indiagrüner mondstein green moonstone indien indiagrüner mondstein green moonstone indien india

mondstein katzenauge orange moonstone cat´s eye indien indiamondstein katzenauge orange moonstone cat´s eye indien indiamondstein katzenauge orange moonstone cat´s eye indien india

brauner mondstein brown moonstonebrauner mondstein brown moonstonebrauner mondstein brown moonstone

Indian moonstones come in many different colours.
Orange varieties are usually termed "pink moonstone" in India.
Note that many show a cat´s eye effect.

sternmondstein star moonstone indien indiasternmondstein star moonstone indien indiasternmondstein star moonstone indien india

Star moonstones are not uncommon. The star is usually 4-rayed although black star moonstone sometimes sport 6 rays (sorry, we don´t have any of those at the moment). 

And then there is alpine moonstone from Austria and Switzerland.
After the discovery of moonstone at the Moerchnerkar Mountain in Tyrol´s Ziller Valley, the notion that alpine moonstone is not of gem quality and thus not worth cutting had to be abandoned

zillertaler mondstein moonstone from ziller valley

Alpine moonstone from the Ziller Valley, Tyrol, Austria

moonstone ziller valley

Some very rare specimens of moonstones from the Ziller Valley (less than 2% of the total find, according to our estimate) show a very interesting phenomenon indeed. If cut in correct orientation, the silvery sheen can be seen on one half of the stone, while the other half appears crystal clear and perfectly transparent. Upon tilting the stone by some degree, the sheen jumps to the other side and the phenomenon reverses.

Handling: moonstone has perfect cleavage and should be set and worn with care. Do not clean ultrasonically!

Worth knowing: moonstone consists of a myriad of lamellae. The blue or silvery sheen is produced by reflection of light at those.

Moonstone shop

Gemmological Properties of Moonstone

Formula:
KAlSi3O8
Crystal system:
monoclinic
Mohs hardness:
6
Specific gravity:
2.53 - 2.56
Refractive index:
1.518 - 1.525
Max. Birefringence:
0.005
Dispersion:
0.012
Pleochroism:
none
Luminiscence:
weak
Lustre:
vitreous
Cleavage:
perfect
Fracture:
uneven, conchoidal
Colour:
colourless, white, orange, grey to black, brown, greenish