Feldspars include the most common minerals found in the Earth's crust. They have two cleavage planes
90 degrees apart so they tend to be rectangular in shape with smooth faces. They are aluminosilicates (mix of aluminum
and silicon tetrahedra) with shared oxygen atoms, though silicon content tends to be higher than chained
mineral groups due to more equal silicon to aluminum ratios. Potassium ions substitute for aluminum or silicon
in potassium / alkali feldspars and sodium and calcium ions substitute in plagioclase feldspar. The most common
gemstones in the feldspars group are moonstone, labradorite and amazonite. There are
varieties of what we call "moonstone" and "sunstone", and all are in the feldspar group.
A common characteristic of feldspars - particularly gemstone quality - is to show a
schiller effect. This is an iridescent play of light off planes within the mineral from microscopic
inclusions. Prehnite is a member of the phyllosilicates group (sheet silicates like mica and serpentine) but with a
higher Mohs hardness (6). Zoisite and vesuvianite are members of the sorosilicates group (sister silicates)
where corners of silica tetrahedra are shared. Tanzanite is a kind of zoisite popular
for its distinctive periwinkle color.
•• a m a z o n i t e
Amazonite takes its name from the Amazon river, though ironically, no actual amazonite deposits are found
along the river. (Deposits of high quality amazonite are found elsewhere in Brazil.)
It is a blue-green variety of microcline, a common potassium feldspar found in granite pegmatites
and metamorphic rocks. The Ilmen mountains in Russia were the primary source for amazonite, but deposits
have been found in the US, Australia and Madagascar as well as Brazil.
•• f e l d s p a r
The stones grouped here are generally granular feldspars with mottled appearance and colors from
lighter beige and grays to pink. They may be potassium-rich alkali feldspars or calcium-rich
•• l a b r a d o r i t e
The schiller effect in labradorite is pronounced, showing lovely blue-green iridescence from the cleavage
surfaces. As labradorite cools, two planes develop from internal chemical separation and it's the light
scattering from these planes that create the iridescence. Labradorite is a middle range feldspar, between
potassium feldspars on one end and sodium / calcium feldspars on the other. The base color is generally blue-gray
or dark-gray but it can also be colorless or white. Most labradorite comes from Madagascar, Finland and
•• m o o n s t o n e
Moonstone gemstones can be potassium-bearing feldspar (orthoclase) and sodium-bearing
feldspar (anorthoclase, oligoclase, sanidine, albite). The gemstone called "rainbow moonstone" is
anorthoclase or sanidine moonstone, which show a strong schiller effect, and the iridescence tends to
be blue-white. The potassium-rich moonstone tends to show more variety in color (cream, grey,
peach) and the schiller effect is not as pronounced.
•• p r e h n i t e
Prehnite is a phyllosilicate (organized in sheets) named for the Dutch officier
Hendrik von Prehn, its discoverer in the late 1700s. Its found lining volcanic
cavities and associated with calcite and mineral veins in granite. Gemstone quality
stones tend to be pale green, but it can also be yellow or gray. It's generally not
found in prismatic form; mostly it appears as short crystals in globular clusters.
Sources include Canada, Australia, Scotland and Germany.
•• s u n s t o n e
Sunstone is an oligoclase (sodium-rich) feldspar with a rich reddish glow from hematite
inclusions. "Oregon sunstone" is included here, but it is actually labradorite
found in Oregon. Sunstone may also be bytownite, a rare sodium / calcium feldspar found
in Canada, that is more yellow-orange in appearance.
•• t a n z a n i t e
Tanzanite is a blue-purple variety of zoisite found almost exclusively on Tanzania.
Vanadium gives the stone its blue color, and tanzanite crystals show pleochroism
(different colors when viewed from different angles). The most valuable tanzanite
is deep violet in color without inclusions, though specimens of this sort are rare.
Tanzanite is often heat-treated to enhance its color and remove any brownish tints,
and this effect is permanent.
•• v e s u v a n i t e
Vesuvanite is a green or chartreuse colored stone that forms in metamorphic regions of
limestone. It's found in marble and frequently accompanied with diopside, grossular garnet
and calcite. It's named for Mt. Vesuvius where it was first discovered.
•• z o i s i t e
When used in jewelry, zoisite is more often ruby in zoisite, which is a green stone with
opaque ruby and black hornblende inclusions. Zoisite that is more pink than green is called
thulite, and this should not be confused with ruby fuchsite, which is opaque ruby ringed with blue
kyanite within fuchsite (mica group) which is more blue-green in color. Ruby in zoisite comes
from Tanzania, where it was found. It forms in metamorphic regions, such as hydrothermal alteration
of igneous rocks, but it can also be found in pegmatites and quartz veins.