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.

"Rainbow Moonstone" (Labradorite) from Madagascar

In German speaking countries "Adular" is sometimes used synonymously to Moonstone, although Adular actually is an Orthoclase variety with a typical crystal habit.

 

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 indiamondstein 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.
The 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

Starmoonstones 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). 

zillertaler mondstein moonstone from ziller valley

Alpine Moonstone from the Ziller Valley, Tyrol, Austria


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.

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