Download Gem Review[1] - Geological Sciences, CMU

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Plate Motion
Plate Tectonics and Rock Cycle
Geological Processes for Gemstones Formation
Magmatic mineral deposits concentrated in igneous rocks;
Hydrothermal/Pegmatitic mineral deposits form in association with
hot water- or gas-rich fluids
Metamoprhogenic mineral deposits concentrated by metamorhism
/ metasomatism
Sedimentary mineral deposits are precipitated from a solution,
typically sea water;
Placer minerals are sorted and distributed by flow of water (or ice);
Residual mineral deposits formed by weathering reactions at the
Schematic model of a fully grown kimberlite pipe
Gem-bearing Pegmatites
Pegmatite is often found in veins (dikes) penetrating
into surrounding continental country rock, although it
can exist as very large bodies also (stock or
batholith size). As with all felsic rocks, is found on
continents. It is one of the last fractionation products
and typically results from partial melting of the lower
crust. http://geollab
Pegmatite containing tourmaline
Cassedanne, J.P. and Roditi. M., 1996. The location, geology and mineralogy of
gem tourmalines in Brazil. Journal of Gemmology, Volume 25, No. 4, pp. 263298.
Tourmaline-bearing pegmatites, which commonly are lens-shaped, are variable
in their strike and dip, and show a range of patterns of zoning and albitization.
Wallrocks are mica schists, quartzites or rarely granite or gneiss, all
Precambrian in age. The pegmatites occur either singly or in fields or clusters
and belong to the Brazilian tectonic cycle (650-450 m.y.).
Five types of gem-bearing pegmatites are described:
1) poorly differentiated, almost homogeneous, with small albitic replacement
2) poorly differentiated with strong albitization;
3) zoned with small albitic replacement bodies;
4) zoned with strong albitization;
5) lithium-rich, very well zoned, with strong albitization.
Spessartite in pegmatite matrix
This photograph shows giant lithium crystals of the mineral spodumene in a pegmatite in the
Black Hills of South Dakota.
From the Greek, spodoumenos, meaning
"burnt to ash".
Spodumene forms as flattened,
prism-shaped crystals that can be
very large, up to 16 meters long
and weighing several tons. The
crystals have fine, vertical ridges
( s t r i a t i o n s ).
Spodumene can be colored white,
gray, yellowish, green (hiddenite),
and pink to purple (kunzite).
Spodumene are an important source of lithium. Hiddenite and
kunzite are used as gemstones even though spodumene
cracks easily. It is found only in granite pegmatites. It can be
found in Scotland, Sweden, Brazil, Burma, and parts of North
Kunzite, the pale pink to lilac gem variety of the mineral
spodumene, is named as a tribute to George Kunz, the
legendary gem scholar, gemologist, and gem buyer. It is often
found in association with morganite and pink tourmaline, the
other popular pink gemstones. Today most kunzite is mined in
B r a z i l , A f g h a n i s t a n , a n d M a d a g a s c a r.
Hiddenite is the green variety of spodumene. It is an
attractive gem mineral, but is rare and for the most part
k n o w n
o n l y
t o
c o l l e c t o r s.
Volcanic Rocks
Peridot in Vesicular Basalt
Hydrothermal Gemstone
Colombian Beryl Deposits
Gemstones as
Products of
Origin of Gem Gravels
Gemstones in dikes or veins
Channel bed rifles
Underwater potholes
Water Fall
Point Bar
Beach Placer
Placer Gem Deposits
Jadeite - is a sodium aluminum pyroxene
that is characterized by its presence in
metamorphic rocks formed at relatively
high pressure. It can form by a reaction of
Albite to produce :
NaAlSi3O8 = NaAlSi2O6 + SiO2
Albite = Jadeite + Quartz
Monkey-puzzle tree or Chilean pine
Amber: the Jurassic Gem
Millions of people learned from the movie that amber, which
is fossilized pine tree sap, is ancient and valuable, like an
a n t i q u e f r o m p r e v i o u s h i s t o r y.
The two main sources of amber on the market today are the
B a l t i c s t a t e s a n d t h e D o m i n i c a n R e p u b l i c.
Amber from the Baltic states is older, and therefore preferred
on the market, but amber from the Dominican Republic is
more likely to have insect inclusions. Prices of amber can
r a n g e f r o m $20 t o $40,000 o r m o r e.
The amber, which was from the Lower Cretaceous period, was mined in the
mountains of Lebanon south of Beirut by Aftim Acra, who has a collection of amber
pieces containing 700 insects, including termites, moths, caterpillars, spiders,
p s e u d o s c o r p i o n s , a n d m i d g e s , w h i c h d o s u c k b l o o d.
Lapis Lazuli is a rock
composed chiefly of the blue
silicate mineral lazurite,
together with calcite and
brassy-coloured pyrite which
are abundant in the poorer
quality material. The vivid
blue of lazurite is cause by the
sulphur that forms an essential
part of its chemistry. Today
lapis lazuli is also mined in
Siberia and in Chile, but
material from these sources
usually contains much calcite.
Lapis Lazuli
Lapis Lazuli is normally a mixture of three minerals:
• lazurite (Na,Ca)8 (AlSiO4 )6 (S,SO4 ,Cl)1-2
• calcite (calcium carbonate, which is white)
• pyrite (an iron sulfide that is white-gold in color)
Lapis lazuli, is a contact metamorphic rock with variable
composition and varying physical properties. It usually
forms by contact metamorphism of limestones.
Lazurite is a felspathoid (somewhat feldspar-like) cubic
(thus, isotropic) hardness: 5.5 S.G. 2.7 - 2.9 vitreous /
greasy luster.
For many centuries the only known
deposits were those at Sar-i-Sang,
1 cm..
in a remote mountain valley in
Badakhshan, Afghanistan. At Sar-iSang the lapis lazuli occurs as a
zone of lenses and veins within
white marble, and grades from deep
to pale blue with some violet and
greenish tints. From here it was
exported to the ancient civilizations
of Egypt and Sumer (Ira), and later
traded throughout the East and into
Europe. These mines are still
Lazurite crystal in quartz matrix
producing the finest quality lapis