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adular

Adular, also known as adularia, is a mineral variety of the alkali feldspar orthoclase, with the chemical composition KAlSi3O8. It is best known for exhibiting adularescence, a soft, milky blue to bluish-lavender glow that seems to float within the stone when it is viewed from different angles. The name adularia derives from Adula, a historic name associated with alpine regions where the mineral was once commonly found, and the term adular is sometimes used to refer to the gemstone variety.

In appearance, adularia is typically colorless, white, or pale gray and displays a pearly to milky luster.

Geologically, adularia forms in low-temperature hydrothermal environments, commonly in veins and pegmatites associated with granitic rocks.

As a gemstone, adularia has historical use in jewelry and lapidary, often marketed under the broader umbrella

It
has
a
hardness
of
about
6–6.5
on
the
Mohs
scale
and
shows
two
directions
of
cleavage
at
or
near
90
degrees,
a
characteristic
of
feldspar
minerals.
Adularescence
results
from
the
internal
microstructure
of
the
crystal
lattice,
which
scatters
light
to
produce
the
distinctive
glow.
Classic
lithologies
include
alpine
hydrothermal
deposits,
though
weight
of
evidence
shows
the
mineral
occurring
in
other
crystalline
rock
contexts
as
well.
of
moonstone
due
to
its
adularescent
effect.
Modern
gemstone
terminology
may
still
cite
adularia
as
a
source
of
moonstone,
though
the
term
moonstone
now
covers
a
broader
range
of
feldspar
varieties.
Adularia
remains
of
interest
to
collectors
for
its
optical
effects
and
its
relevance
to
feldspar
mineralogy.