Moonstone: Gemstone Information
The variety name moonstone is used to describe an optical effect and unlike most variety names it is not confined to a single species. Moonstone gets its name from its appearance which is likened to the reflected shine produced by moonlight. It is normally characterized by a milky white to bluish shimmering sheen effect that seems to float
across the convex surface of the stone as it is turned and moved: the more intense the colour and the larger and more transparent the stone, the more valuable the gem. Top quality fine blue moonstones show an incredible ‘three dimensional’ depth of colour. This effect has been variously termed ‘adularescence’ or ‘peristerescence’ depending upon the dominant species. The term ‘schiller’ has been used to describe both of these.
The term ‘adularescence’ is derived from the variety of orthoclase feldspar, known as adularia. Peisterescence is derived from peristerite, a variety of plagioclase feldspar of approximate composition An1–An17 (see explanation of Figure 3 – Peristerescence). Thus moonstones, which are usually cut in a smooth-domed cabochon shape to maximize the effect, can be either K-feldspar moonstones (normally adularia) or plagioclase moonstones (normally peristerite): each having RI and SG commensurate with the dominant species, normally orthoclase or albite/oligoclase. The K-feldspar moonstones have lower physical constants with RI around 1.518–1.526 and SG
between 2.56 and 2.59 whilst the physical constants of peristerite moonstones have been reported around 1.531–1.539 for RI and 2.63 for SG with DR 0.008.
Interference and Iridescence
When a ray of white light falls upon a transparent medium, some of the light is reflected, whilst the remainder is refracted within the optically denser medium. When the refracted ray meets another surface, such as exsolved lamellae, some is reflected back from the surface and some is refracted, penetrating deeper into the medium where, at the next interface, the process is repeated.
The speed of light is retarded in the optically denser medium. Each constituent colour of the white light is dispersed such that each colour follows a different path and is retarded to a different degree. These
will be reflected from the various interfaces. As they exit the medium similar colours will try and recombine but because they followed different paths and experienced different retardation they may exhibit phase variance. Where there is no phase variance the rays reinforce each other (constructive interference). Where there is phase variance the rays will have a cancelling effect on each other (destructive interference).
Thus the emerging combined rays may not exit as white light but exhibit colour. The colour effects caused by this phenomenon are termed ‘iridescence’.
Light scattered from exceptionally low concentrations of very fine colloidal particles in a transparent medium produces a milky white to bluish body colour that is termed opalescence (not to be confused with the play of colour in opal) such as seen in unhomogenized milk or in some moonstone imitations made by heat-treating synthetic spinel (presumably to precipitate a second phase). This term has also been applied to moonstone to describe the milky or pearly appearance. It has been suggested therefore that the white to bluish moonstone effect is
caused by light scattering from very fine particles. In support of this it has been reported that in feldspars, where exsolved areas are irregular and small compared to the wavelength of light, white to bluish colours may be produced by incoherent light scattering such as the Tyndall effect. Further research has shown that very fine exsolution lamellae can form along planes other than that normally associated with exsolution in peristerite. One such plane has a periodicity that has been shown to correlate with iridescence. It is therefore possible that more than one phenomenon contributes to the moonstone effect.
The most prominent source is in the gravels and pegmatites of the Dumbara district, Sri Lanka. Many gem moonstones come from India, Brazil and Madagascar. Other localities include Switzerland, Tanzania, Pennsylvania, New Mexico Colorado, Indiana, New York, North Carolina, Pennsylvania, Wisconsin and Virginia in US, and Myanmar, Canada, Australia and Russia.
Moonstones come in a variety of colours. The body colour can range from colourless to grey, brown, yellow, pale orange or peach, green, plum-blue, reddish or pink. Inclusions can effect the overall body colour; goethite inclusions, for example, imparting red colouration. The clarity ranges from transparent to translucent. The best moonstone has a blue sheen, perfect clarity and a colourless body colour. This material has become increasingly rare and the term ‘blue-sheen’ has been stretched to include gems of lesser quality. Consequently the term
‘Royal Blue Moonstone’ has been introduced in an attempt to preserve the quality differential.
The K-feldspar moonstone has no characteristic absorption spectrum but may show weak bluish fluorescence (LWUV) and weak orange (SWUV).
Commonly, especially in Sri Lankan material; abundant short parallel stress-related elliptical incipient cleavages at right angles to a ‘spine’. Stress-related lath-like cracks, often in pairs, with numerous branching
cracks that extend for a short distance and taper off obliquely such that they resemble a multi-legged insect and consequently often termed ‘centipede inclusions’ or occasionally ‘Chinese aeroplanes’ Negative rectangular inclusions that are thought to possibly be stress-related have been reported. Some Myanmar stones may show orientated needle-like inclusions which when abundant may well produce a cat’s eye effect. Rare bamboo-like inclusions in K-feldspar moonstone from Sri Lanka arise from sub-parallel tube-like inclusions (stalks) with
intersecting cleavage cracks (nodes).
• Rainbow moonstone (see labradorite) An iridescent labradorite from Madagascar is being marketed as
rainbow moonstone. The material has a slightly grey body colour and dark inclusions but is otherwise similar to the rainbow moonstone from southern India
• Norwegian moonstone (see labradorite)
• Black moonstone (see labradorite).
Optical Effects: Chatoyancy, Asterism, Colour Change, ADR Sometimes moonstone will have a cat’s eye as well as a sheen and occasionally a four-rayed star may be present. Brownish grey moonstone from Sri Lanka with RI 1.53 and SG 2.6, SWUV red fluorescence, slightly translucent with silvery schiller shows a combination of both Cat’s eye and six-rayed star, the latter attributed to micro-inclusions of hematite or ilmenite. Chatoyant moonstone with a typical darker orange body colour is reported from a deposit near Chennai, India.
Moonstone: Gemstone Information