To all interested in the subject.
I will try to answer several questions in one shot if I can.
What is adularescence?
It is an optical phenomena, which should only be used in conjunction
with precious moonstone, also knows as adularia. The effect is due to
very specific crystallization pattern. Precious moonstone is far more
expensive than perusal of gem dealers catalogues might suggest. What
is sold under the name of moonstone nowadays is simply a heavily
included orthoclase. That you can cut any way you want.
Why moonstone (adularia) must be cut en cabochon to observe
Precious moonstone is micro layers of orthoclase interspersed with
micro layers of albite. Both minerals belong to feldspar group. The
only difference between them is that for albite to form, potassium
must be present; but orthoclase require sodium. Both metals commonly
present in hydrothermal solutions, and once solution begins to cool
off, both mineral begin to exsolve. Since exsolution of let us say
orthoclase depletes surrounding from sodium, it creates potassium
rich environment and albite would be formed next. The process repeats
itself over and over again creating micro-layered structure.
When such stone cut en cabochon and base of cabochon is oriented
parallel to the layers, a light ray is subsequently bent by each and
every layer until the angle of incidence becomes so shallow that it
reflects from some layer deep within the stone. When this happens ray
of light becomes trapped in one of these layers and will exit on the
side of gemstone by bouncing back and forth. What we perceive as
adularescence is interaction of ray of light with layer boundary. We
cannot see just single interaction, so to produce cummulitive effect
we cut gemstone en cabochon. It creates multiple entry points with
different angles. We can say that cabochon gathers light from
If precious moonstone is faceted, the only facet to replicate the
effect would be the facet parallel to the layers. There are number of
problems with that idea.
If facet orientation is slightly off, the effect would be weak or
none at all. (In cabochon this is compensated by spherical shape) To
get a facet in such precise alignment is a very tall order indeed. I
would suggest that it is beyond ability of most of cutters.
Even if a cutter somehow manages to orient that single facet
correctly, the effect would be barely noticeable, if at all. It would
be so weak that it will be totally obscured by other optical
phenomena. I can conclude by saying that what is called adularescence
in faceted orthoclase ( we should not call it moonstone ) is not the
adularescence, but simply a reflection of light from inclusions and
other crystal irregularities.