Moonstone is a member of the feldspar family and is admired for its ‘floating light’ optical property called adularesence or schiller.  Once believed to be solidified moonlight, its common colours are white, colourless, grey, peach, brown and blue.  Typically most ‘trade’ moonstone refers to the translucent white to slightly greyish blue variety and is quite inexpensive. 

The most highly valued moonstones are the high transparency, inclusion-free, colourless type with a strong blue adularesence known as ‘blue top’ moonstone.  Another form of rare and valuable feldspar is ‘rainbow labradorite’.   Gemmologically this material is a form of colourless labradorite but is known to the jewellery trade as ‘rainbow moonstone’.  It looks just like the colourless blue top moonstone, however, instead of a straight blue adularesence, rainbow moonstone like its name suggests displays the most intriguing adularescent spectral colours.  Important mining locations are India, Sri Lanka and Myanmar (Burma).       

Mohs’ Hardness: 6-6.5
Specific Gravity: 2.54-2.63
Refractive Index: 1.520-1.570
Birefringence: 0.005
Optic Sign & Character: Biaxial Negative
ICA Code: N
Treatments: None
Incidence: N/A
Purpose: N/A
Durability: Poor to good
Special Care: Pressure sensitive – Avoid ultrasonic
Synthetics: None
Possible Imitations: Glass