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Spinel (MgAl2O4) is a much maligned gem mineral that is often
thought of as a corundum simulant because in red and blue
varieties it can closely resemble ruby or sapphire. It is, however,
an outstanding gem material in its own right; it is hard (8),
possesses an intermediate to high R.I., shows good dispersion,
and is available in a wide variety of colors. Many of the worlds
most famous large "rubies" are, in fact, red spinel (e.g. Black
Princes' Ruby and Timur Ruby, both in British Crown Jewels).
Such stones were once referred to as Balas Ruby, a term that is
not used today but is synonymous with red spinel. The confusion
is well-founded; the finest red spinels come from the same gem
gravel deposits in Myanmar that are renowned for "Burma" ruby,
and both share somewhat similar optical and physical properties.
British Crown Jewels
The Black Prince’s Ruby now is
mounted in the front of the Imperial
State Crown, just above the famous
Cullinan II Diamond. It is a huge,
s e m i - p o l i s h e d o c t a h e d r o n.**
Sitwell15 states that the stone is
backed by a gold foil, as were many
ancient g ems, to imp rov e its
brilliance . This has not been
removed for fear of damaging the
gem. The stone measures some two
inches (5.08 cm) in length and is of
proportionate width.14 Its exact
weight is unknown, but estimates put
Spinel, like garnet and tourmaline, is a mineral name that refers
to a group of minerals all having the same crystal structure.
Members within the group differ by containing varying amounts
of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the
crystal lattice. Probably the most familiar minerals that have the
spinel crystal structure are the ore minerals magnetite
(Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a
gem material. Gem spinel is usually quite close in composition
to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr
that act as chromophores and account for the wide range of
possible colors. Red and pink spinel is colored by trace
quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr,
and Co (rare) are also present in some blue, violet and purple
Pain Pyit, Mogok, Myanmar (MgAl2O4)
• Mogok Stone Tract
• Source of most of the famous large red spinels; finest red
and pink spinel found here.
• Source is metamorphosed limestone and its weathering
products (gem gravels).
• Grey-blue or black material may show 6- or 4-rayed stars.
• Magnetite inclusions are characteristic
Parker mine,Notre Dame de Laus,Quebec,Canada
Sri Lanka, Thailand, Cambodia
• Same gem gravels that produce corundum.
• Most Sri Lankan spinels are said to be paler (color
"more subdued"; "lighter tone") and less included than
their Burmese counterparts.
Table I. A selection of spinels
Spinel itself, base for natural gemstones
Gahnite, a transparent diamagnetic spinel
Hercynite, a classical paramagnet
Maghemite, a natural material for magnetic recording
Chromite, the chrome ore of Rhodesia
Hausmannite, a natural tetragonal spinel
Table I. (continued)
Magnetite, the ancient navigator’s lodestone
Greigite, a ferrimagnetic semimetal
Trevorite, a ferrimagnetic semiconductor
Franklinite, the paramagnetic ferrite
Carrollite, a natural metallic spinel
Ulv?spinel, with giant magnetostrictive properties
The high-pressure spinel polymorph of forsterite
(olivine), thought to comprise the earth’s inner mantle
Spinel Compounds: Background & Historical Perspective
Kurt E. Sickafus and Richard Hughes
Submitted to the Journal of the American Ceramic Society, November, 1999
There are reports of a
recent find of a large
deposit of fine pink
spinel in the Pamir Mts.,
T a j i k i s t a n.
Reds, brownish to plum-red, lilac, violet, and blue.
Contain inclusions of green amphibole and rutile
Zircon is zirconium silicate, ZrSiO4, and is notable as a gem
mineral for its high R.I., high dispersion, and subadamantine
luster. Prior to advent of modern gem synthetics, colorless zircon
was the stone of choice as a diamond simulant because of its
diamond-like optical properties. Cubic Zirconia is not synthetic
zircon but is zirconium oxide (ZrO2). Nearly all zircon contains
trace quantities of U, Th, and Pb. The natural radioactive decay
of uranium and thorium within zircon damages the crystal lattice,
sometimes to the extent that crystals become virtually
amorphous, and results in an atomic structural state that is
referred to as "metamict". Such damage can render crystals
darker and cloudier, and lowers the measurable optical and
p h y s i c a l
p r o p e r t i e s.
Zircon is present in trace quantities in nearly all intrusive
igneous rocks, particularly granites, where it occurs as very
small crystals. Gem zircon is nearly restricted in occurrence to
pegmatites or to quartz-poor, alkalic intrusive rock (e.g.
syenites). The gem gravels of Thailand are the most important
commercial source, followed by production from gem gravels in
M y a n m a r
a n d S r i
L a n k a.
Gem-quality zircon is normally brown, but natural blues,
oranges, greens, yellows and colorless stones are known. Color
is the product of color centers and/or trace quantities of Fe.
Nearly any color of the rainbow can and is produced by heat
treatment of brown zircon. Heat treatment of zircon is
Australia: zircon is extracted from sands. The most important source is beach
deposits on the coast of Queensland. These produce 250,000 tons
Austria, Tyrol region: zircon has been found associated with pink titanite
Brazil, Minas Gerais: has produced light-green transparent crystals. J
Canada, Renfrew County, Ontario: crystals up to one foot long and weighing 7kg
h a v e
b e e n
o b s e r v e d.
Germany, Eifel Mountains: as small druses of colorless zircons in trachyte
Russia, Ilmen Mountains: crystals up to 3.5kg of prismatic habit occur in veins in
granite associated with orthoclase, mica and apatite and in the
Ural Mountains where crystals up to 5cm have been found.
Thailand, Cambodia, South Vietnam: are the main producers of gem zircon. It
occurs as water-worn pebbles in gem gravels at depths usually < 10 feet.
Sri Lankan Cut Zircon
Thailand (most of worlds present production), Sri Lanka,
Burma, Cambodia, Laos and Vietnam. All from gem
gravel deposits or gem alluvium. Produced as a byproduct
c o r u n d u m
m i n i n g.
"Mud Tank" Zircon Field, Harts Range,
Northwest Territory, Australia
Zircon from the
4.14 ct., 11.3 x 8.3 x 5.9 mm, VVS
The Harts Range is a low mountain
chain approximately 70 km northeast of Alice Springs and extending
west for over 100 km. The region
is arid and supports little vegetation
(See right) The mountains are
composed of volcanic and strongly
metamorphosed sedimentary rock
that has produced significant
amounts of gold. Pegmatites
scattered throughout the region have
been commercially mined for mica
and beryl. The region has also
produced small amounts of gem
quality aquamarine, tourmaline,
g a r n e t , a n d i o l i t e.
Approximately 150 km north-northeast of Alice
Springs is the Mud Tank gemfield (Named for the
local bore, wind pump, and water storage
reservoir), famous for gem quality zircon . At this
site zircon, massive chunks of yellow-green
apatite, and magnetite have weathered from the
bedrock and can be found by screening the thin
layer of topsoil and washing the gravel (See one
day's yield above, right). The zircon display a
variety of colors; brown, pink, shades of purple,
yellow, and colorless (See adjacent photo).
Approximately 20% of the zircon from this
particular area is suitable for faceting. Because of
the high dispersion of zircon, this material