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Leopard Opal: Play-of-Color Opal in Vesicular Basalt from Zimapán, Hidalgo State, Mexico

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"Leopard opal" consists of vesicular basalt impregnated with play-of-color opal, and is known only from Zimapán, Hidalgo State, Mexico. The formation of this ornamental stone was made possible by an abundance of silica derived from the chemical breakdown of overlying volcanic ash layers, the permeability of the underlying basalt, and the presence of pores in the basalt of an aesthetically pleasing size. The even distribution and small size of the opal-filled vesicles makes the rock attractive when cut or carved and polished. Veinlets and irregular masses of play-of-color opal showing various bodycolors (red, white, and colorless to pale blue) have also been deposited along joints and fractures within the basalt flow. This opal deposit, which may have been worked in pre-Columbian times, has been explored only by a number of small test pits in recent years, and significant potential remains for its future development.
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ne of the authors (ARZ) spent much of his
youth as a gambusino, or prospector, explor-
ing remote areas of the Mexican country-
side on horseback. In 1965, while investigating some
bushes where a fox was hiding, he noticed flashes of
color in a lump of vesicular basalt that proved to con-
tain opal. Further prospecting led to the discovery of
the deposit itself higher on the hillside. However, it is
possible he had only rediscovered it, as an old shal-
low trench suggested that the deposit could have
been worked for opal much earlier, perhaps by pre-
Columbian inhabitants of the region. (There are no
records of opal having been found in this area by the
Spaniards, nor are the authors aware of any recent
opal mining other than that of ARZ.) Shortly there-
after (in 1965), ARZ staked the mining claims that
are known today as Gemma and Desiré.
Because of the striking spotted appearance of the
opal against its black basaltic host, this unique mate-
rial became known as “Leopard opal.” Word spread,
and soon the material attracted the attention of an
American, Foster Conton, who visited the prospect
and returned to the U.S. with a 6 kg sample given to
him by ARZ. He gave the specimen to Albert Eugene
Upton, who after months of research and study
decided to have the stone carved into an art piece
that would have historical significance for Mexico: a
likeness of Cuauhtémoc, the last emperor of the
Aztecs (figure 1). Artisan Rafael Tapia of Taxco,
Mexico, was commissioned to undertake the carving,
which took seven months to complete and had a
final weight of 3.375 kg. The silver headdress and
mounting were made by Alejandro Gómez. The
complete statue stands approximately 48 cm high
and weighs a total of 8.2 kg (E. Littig, pers. comm.,
1999). Photos of this carving were published in
Lapidary Journal (Leipner, 1969), which also adver-
tised rough and cabochon-cut pieces of “black matrix
opal” from this deposit. In 1970, Mr. Upton donated
the carving to Sacred Heart College (now Newman
University), in Wichita, Kansas. Apart from some
samples cut and polished locally in Mexico or by for-
eign lapidaries over the years (e.g., figure 2), and an
236 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
LEOPARD OPAL: PLAY-OF-COLOR
OPAL IN VESICULAR BASALT FROM
ZIMAPÁN, HIDALGO STATE, MEXICO
Robert Raymond Coenraads and Alfonso Rosas Zenil
See end of article for About the Authors and Acknowledgments.
GEMS & GEMOLOGY, Vol. 42, No. 4, pp. 236–246.
© 2006 Gemological Institute of America
“Leopard opal” consists of vesicular basalt impregnated with play-of-color opal, and is known
only from Zimapán, Hidalgo State, Mexico. The formation of this ornamental stone was made
possible by an abundance of silica derived from the chemical breakdown of overlying volcanic
ash layers, the permeability of the underlying basalt, and the presence of pores in the basalt of
an aesthetically pleasing size. The even distribution and small size of the opal-filled vesicles
makes the rock attractive when cut or carved and polished. Veinlets and irregular masses of
play-of-color opal showing various bodycolors (red, white, and colorless to pale blue) have
also been deposited along joints and fractures within the basalt flow. This opal deposit, which
may have been worked in pre-Columbian times, has been explored only by a number of small
test pits in recent years, and significant potential remains for its future development.
O
appearance of the material at the 1996 Tucson gem
shows (Johnson and Koivula, 1996), there has been
little public exposure of Leopard opal, and the deposit
has lain in relative obscurity for the last several years.
This is the only known opal deposit in the
Zimapán area. The historical significance and geo-
logic setting of the site, as well as the small-scale
mining activities carried on there, have not been
described previously in the literature. To date it has
been explored only by a number of small test pits,
but the authors believe significant potential
remains for its future development.
A History of Mexican Opal. Long before its redis-
covery in modern times, opal was mined and
appreciated as a gem material by the pre-Columbian
LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006 237
Figure 1. Leopard opal,
vesicular basalt impreg-
nated with play-of-color
opal, can be polished
into cabochons for use
in jewelry or employed
as a carving material.
This bust (3.375 kg)
shows the likeness of
Cuauhtémoc, the last
emperor of the Aztecs.
The carving is by Rafael
Tapia, and the silver
work is by Alejandro
Gómez; it is part of the
Newman University
collection in Wichita,
Kansas. Photo by
Charles Rasico, cour-
tesy of Newman
University.
peoples of Mexico and Central America. There are
two terms in Nahuatl, the Aztec language, that are
used to describe opal: quetza litzle pyolitli, meaning
“stone which changes color in movement” (or “bird
of paradise stone”) and huitzitziltecpatl, “stone like
a bird of a thousand colors” (or “hummingbird
stone”). Some of these Mexican opals were taken to
Europe and the present United States by the
Spaniards in the early 16th century (White, 1998).
The Spanish monarchy, however, had more of an
interest in finding Mexican gold, and opal is not men-
tioned on the invoices of precious objects sent by the
Spanish conquistador Hernán Cortés to Charles V of
Spain during this period (Leechman, 1961).
Eventually, many of the pre-Columbian opal
mines were closed and their locations lost. It is
interesting to note that Zimapán is indicated as
the source of many old opal specimens in muse-
ums around the world (see, e.g., Leechman, 1961;
Heylmun, 1984b). One such example is a ring con-
taining a “fire opal of Zimapán” (Ball, 1931) worn
by Antonio Eusebio de Cubero in a 17th century
portrait by Diego Velázquez.
In the mid-1800s, the opal deposits in the state
of Querétaro were rediscovered (see, e.g., Koivula et
al., 1983). Numerous small open-cut mines began
operating in the district (Heylmun, 1983a), and the
capital, Querétaro City, became Mexico’s most
important cutting and polishing center (Webster,
238 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
Figure 3. The Leopard opal deposit is located south-
west of the town of Zimapán in Hidalgo State, Mexico
(after CETENAL, 1972).
Figure 2. This pre-classical-style jaguar head (44.76 ×
42.45 ×11.30 mm) carved from Leopard opal was
fashioned by Kevin Lane Smith of Tucson, Arizona.
Photo by Maha Calderon.
LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006 239
1983). The region has made Mexico famous for its
fire opal, an orange-red (fiery) opal that often dis-
plays play-of-color. Most Mexican fire opal is open-
pit mined from hard pink-to-red Cenozoic rhyolite
using labor-intensive hand methods (see, e.g.,
Mallory, 1969a,b; Zeitner, 1979).
In the last 150 years, opal has been found
throughout Mexico’s Central Volcanic Belt (Heyl-
mun, 1984a) in the states of Querétaro, Hidalgo,
Guanajuato, San Luis Potosí, Guerrero, Michoacán,
Jalisco, and Nayarit. Detailed locality maps and
descriptions of the mines were published in a series
of articles by Heylmun (1983a,b,c). The wide vari-
ety of opal types found in Mexico and their local
nomenclature are described by Heylmun (1984a).
Other Mexican opals, in addition to fire opals,
show play-of-color, and some of these are legendary
among early Mexican production. The Aztec Sun
God opal, a 36 ×34 ×15 mm, 94.78 ct stone, is
believed to originate from Mexico; it has a transpar-
ent pale blue bodycolor and displays blue, green,
yellow, and red play-of-color (White, 1998). This
opal is carved into an image of the sun with a
human face. Another notable opal, El Águila
Azteca (The Aztec Eagle), was discovered in an
excavation in Mexico City around 1863 and is
believed to have been part of the treasures of the
Aztec ruler Moctezuma II (1502–1520). This 32 ct
eagle’s head was said to “exhibit an infinite series
of prismatic colors from a pale lavender to deep
ruby red” (White, 1998, p. 46); its present where-
abouts are unknown.
LOCATION
The Leopard opal deposit is located 14 km south-
west of the town of Zimapán in Hidalgo State at
20°41.8N, 99°27.7W (figure 3). The site is not
open to the public, and permission to visit must be
obtained from the second author (ARZ). The mine
is accessed by a rough dirt track that winds its way
to the base of La Piedra Grande (The Big Rock, or
Tandhé in the local Otomhé language), a peak that
forms part of a NW-SE trending range. The last
kilometer of the track must be negotiated using a
4-wheel drive vehicle or on foot. The mine is locat-
ed at an elevation of about 2000 m, at a break in
slope caused by a change in rock type (figures 4 and
5). A number of small pits and trenches that have
produced opal-bearing material may be seen here
lying in a trend that follows the geologic contact
(figure 6).
GEOLOGIC SETTING
The geology of the region around Zimapán was
described by Carrillo and Suter (1982). The opal
deposit is hosted by a sequence of undifferentiated
Tertiary-Quaternary age volcanic rocks. On the
northeastern slopes of the range (again, see figures 4
and 5), the lower portion of the sequence consists of
intercalated lava flows, the most dominant being a
massive red-brown quartz porphyry that is clearly
visible in the field as a cliff-forming unit. Above
these lava flows, and extending to the top of the
range, is a series of light-colored units ranging from
fine ash-fall tuffs and breccias to layers of pyroclas-
tic blocks (solid rocks blown out of an erupting vol-
cano) up to 50 cm across. The entire volcanic
sequence has been tilted about 20° southwest and
has been offset locally by faulting.
At the mine site, a discontinuous unit of vesicu-
lar basalt is found along the contact between the
lava flows and overlying pyroclastic layers (again,
see figure 5). Vesicular basalt forms when water
vapor and gases such as carbon dioxide cannot
escape quickly enough from the cooling lava, thus
leaving open cavities, or vesicles. The basalt erupt-
ed as a lava flow that probably filled a small valley.
The vesicles are typically stretched into cylindrical
shapes, formed as the hardening (but still plastic)
lava continued to flow downhill. There are more
Figure 4. The Leopard opal workings are located at
2000 m on a ridge that projects eastward from the
side of the peak known as La Piedra Grande or
Tandhé (right, background). The workings (yellow
arrow) are located along the contact between the
softer white tuffs and breccias above and the harder
dark vesicular basalts below. The road leading to the
mine can be seen as a light line on the darker unit.
Photo by R. Coenraads.
gas bubbles toward the top of the lava flow because
the gases that slowly rose through the still-molten
interior remained trapped below the hardened
upper skin. Here, the vesicles are sufficiently
numerous that the rock has become quite porous
and permeable to groundwater. The basalt was, in
turn, overlain by ash-fall tuff and breccia units
deposited during a series of explosive volcanic erup-
tions. The ash-fall units are also very porous, and
the feldspar and silica glass–rich portions have
largely weathered to clay. The presence of abun-
dant cryptocrystalline silica films coating the sur-
faces of faults and fractures in the lower units sug-
gests that the reaction associated with this weath-
ering process liberated significant quantities of sili-
ca, which migrated downward and precipitated into
any available open space.
The elongated vesicles or gas cavities in the
basalt are filled with transparent colorless opal to
semi-opaque opal of various bodycolors ranging
from white to pink or pale blue. In some hand speci-
mens, the different bodycolors appear to run in par-
allel bands or patches (figure 7). The opal displays
play-of-color ranging from red to violet.
Together with the vesicle-filling opal, two dis-
tinct generations of play-of-color opal are also
found filling fractures in the basalt. These are a
translucent reddish orange opal (figure 8) typical of
the fire opal for which Mexico is renowned, and a
colorless, transparent to pale blue translucent vari-
ety similar to the opal found in the basalt vesicles.
EXPLORATION AND MINING
Since 1965, occasional activities by one of the
authors (ARZ) and his family have left several small
shallow exploration and mining pits spread out
along the geologic contact for a distance of several
hundred meters. The section of the old trench found
by ARZ, and thought by the authors to be pre-
Columbian workings (again, see figure 6, right), also
follows the contact. The excavations by ARZ show
that opal has permeated the top of the basalt flow,
but with a patchy and discontinuous distribution
parallel to the contact. Exposures in the pit shown
in figure 6, left, reveal that the concentration of opal
in vesicles and along fractures is highest at the
upper surface of the basalt, which is in direct con-
tact with the overlying tuffs. The concentration of
opal falls off rapidly within several meters. At
greater distances from the contact, the basalt vesi-
cles are empty. Approximately 1,500 kg of Leopard
opal have been removed from these excavations
using simple hand-mining techniques, and almost
all of this production has been sold in the U.S. The
6 kg piece used for the Cuauhtémoc carving (again,
see figure 1) is the largest found to date. Most pieces
recovered weigh less than 1 kg.
240 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
Figure 5. The zone of opalization occurs within the top few meters of a vesicular basalt that is overlain by ash and
breccia layers, as seen in the cross-section on the left. The entire sequence dips about 20° southwest. In the photo
on the right, looking across the valley to the north of the exploration area, the contact between the well-bedded,
whitish volcanic tuffs and breccias, and the underlying darker, featureless basalts is clearly visible. The bedding
and the contact are folded and displaced by faulting (visible as diagonal lines). Photo by R. Coenraads.
Fault
Fault
Contact
The fire opal and pale blue opal is found in thin
veins (again, see figure 8) crisscrossing the deposit.
The veins thicken in places up to about 2 cm, but
the opal tends to fragment into small pieces when
dug out. Most is milky and sun-damaged, but it
recovers its original appearance and play-of-color
when wet. To date, there has been no exploration
for this vein opal at depth, where unweathered
material may exist. It is possible that this was the
type of opal being sought in the pre-Columbian
workings, as it closely matches descriptions of the
above-mentioned historic pieces, and it is the only
known locality of such material in the Zimapán
area.
More extensive exploration and deeper mining
of the geologic contact in the vicinity of the earlier
workings could yield good quantities of opal. The
geologic contact also needs to be followed along
strike, beyond the known area of opal occurrence,
where it may have the potential to produce similar
opal-bearing material on adjacent hillsides.
LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006 241
Figure 7. This sample of vesicular basalt (15 cm)
was collected near the contact of the lava flow and
overlying tuffs and breccias. Most of the vesicles in
this sample are filled with transparent-to-translucent
opal, and a distinct zonation in opal bodycolor is vis-
ible. Photo by R. Coenraads.
Figure 6. At left, one of a series of mining pits and trenches is located along the contact between the white tuffs
and the vesicular basalt. The top of the black basalt lava flow has been exposed, with the hollows and depressions
of its surface still filled by white tuff. The opal is concentrated in the vesicles and fractures in the top of this basalt
flow. In the photo on the right, the surface expression of the contact between the white tuff (left) and the darker
basalt (right) is clearly visible, and a shallow depression can be seen running along the contact. This trench may
represent pre-Columbian workings of this opal deposit, as the types of opal recovered here are consistent with
those used in pre-Columbian pieces. Photos by R. Coenraads.
MATERIALS AND METHODS
About 2 kg of Leopard opal and chips of opal from
the veins were collected by the authors for study.
The material, shown in figure 9, includes 9 g of red-
dish orange fire opal, 5 g of colorless to pale blue opal,
and a 6.87 ct cabochon from the personal collection
of ARZ. Prior to gemological testing, flat faces were
polished on several of the vein opal chips, and two
pieces of the rough Leopard opal were ground smooth
and polished on one side, yielding final weights of
205 ct and 100 ct. These two Leopard opal samples
were prepared according to the guidelines recom-
mended for this porous material (Leipner, 1969). The
material was soaked in water prior to slabbing, sawn
using a water-soluble mixture (i.e., not oil), and was
polished with darker polishes rather than light-col-
ored polishes that are difficult to remove from the
pores. Excessive wheel speed and pressure were
avoided, as these can generate heat that might dam-
age the opal. The 6.87 ct cabochon showed minor
undercutting of the opal spots, since opal, with a
Mohs hardness of slightly less than 6, is softer than
the feldspar of the basalt matrix, which has a hard-
ness of 6–6.5. Specimens with a more careful sample
preparation showed no variation in surface relief.
All samples were examined with a 45×binocular
microscope and viewed in a darkened room with a
Raytech short- and long-wave UV lamp. Spot refrac-
tive index (R.I.) readings were conducted using a
Topcon refractometer, and specific gravity (S.G.)
was determined for opal pieces without matrix
using an Oertling R42 hydrostatic balance. A stan-
dard thin section (0.03 mm) of the Leopard opal was
cut at New South Wales University in Sydney for
study with a polarizing petrologic microscope.
Small pieces of fire opal and colorless “crystal”
opal from veins within the Leopard opal deposit, both
exhibiting good play-of-color, were crushed for X-ray
diffraction (XRD) analysis at the Australian Museum
laboratory in Sydney, and the results were compared
to standard scans by Diffraction Technology,
Canberra, Australia. Other samples from the veins
were etched in hydrofluoric acid vapor for times vary-
ing between 90 seconds and several minutes to reveal
their internal structure, gold coated, and then imaged
using the scanning electron microscope (SEM) at the
University of Technology, Sydney.
RESULTS
In hand specimens, the black basalt takes a high
polish, thereby providing a good background for the
242 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
Figure 9. Samples of opal from Zimapán used for this
study include a 205 ct polished sample (top), a 100 ct
polished sample (center), and a 6.87 ct cabochon (cen-
ter left) of Leopard opal; 9 g of fire opal (bottom right);
and 5 g of colorless to pale blue opal (bottom left).
Photo by R. Coenraads.
Figure 8. Translucent opal displaying an orange-red
bodycolor (fire opal) fills cracks and fractures in the
vesicular basalt near the contact. The opal appears to
have deposited at the narrowing ends of fractures;
some of it shows play-of-color. A Mexican coin (2.5
cm) is provided for scale. Photo by R. Coenraads.
“leopard spots” of play-of-color opal. The loupe and
binocular microscope provide conclusive identifica-
tion of the material. In figure 10, the 205 ct sample
shows a distinct elongation and orientation of its
opal-filled vesicles, and also a marked opal bodycol-
or zonation from blue to white, similar to that visi-
ble in some of the rough hand specimens (again, see
figure 7). Figure 11 shows several views of the vesi-
cles of the 100 ct sample; these are filled with trans-
parent opal displaying play-of-color domains that
are continuous over several vesicles. Detailed exam-
ination (figure 11, center) revealed that the vesicles
are highly irregular in shape. Although some have
remained empty (see the bottom right corner of the
photo), most appear connected to one another by
channels along which the silica-bearing fluids could
migrate. The inside walls of the vesicles are white,
which suggests that a thin coating of another miner-
al was deposited before the vesicles were filled with
opal (again, see figure 11). At higher magnification
(figure 11, right), long thin colorless feldspar crystals
are visible in the dark basalt matrix.
The thin section (figure 12) revealed that the
basalt in this specimen is unweathered and consists
of abundant microscopic (0.1–0.2 mm) euhedral
LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006 243
Figure 10. This detail from the 205 ct polished sample
in figure 9 displays oriented (bottom right to top left)
irregularly shaped vesicles filled with opal that varies
in bodycolor from blue (bottom) to yellow, pink, and
white (top). Photo by Graham Henry; field of view is
18 mm high.
Figure 11. In this series of photomicrographs, the 100 ct polished sample in figure 9 also shows oriented vesi-
cles filled with colorless opal displaying play-of-color. A domain displaying red play-of-color continues over
two vesicles (left). At higher magnification (center), this sample shows an irregular-shaped vesicle that illus-
trates the interconnected channelways through which the silica-bearing fluids originally migrated. At right,
the basalt host reveals numerous elongated transparent feldspar crystals oriented parallel to the direction of
elongation of the vesicles. These would have been parallel to the original flow. Photomicrographs by Graham
Henry; height of field of view is 11 mm (left), 4 mm (center), 3 mm (right).
244 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
feldspar crystals; these are oriented more-or-less par-
allel to the direction of the original lava flow (figure
12, center). The vesicles vary in size up to ~1–3 mm,
and are also elongated in the direction of flow
(again, see figure 12, center). When viewed between
crossed polarizers, some of the opal in the vesicles
remained dark upon rotation through 360°, as
would be expected from an amorphous material,
while some displayed diffraction colors (figure 12,
center and right). Such colors are typical in thin sec-
tions of opal showing play-of-color (R. Flossman,
pers. comm., 1999) because of the pseudo-crys-
talline nature of the material caused by the regular
arrangement of its silica spheres. Consistent colors
are often visible over several adjacent vesicles, indi-
cating that the orderly deposition and arrangement
of silica spheres proceeded unimpeded within the
framework of the host basalt.
Viewing Leopard opal with UV radiation high-
lights the inhomogeneous nature of the material.
The basalt matrix is inert to UV, while the opal
stands out as bluish white spots (stronger under
long-wave than short-wave UV) against the black
background (figure 13).
Determinations of S.G. and R.I. are not meaning-
ful for Leopard opal, since they will invariably
reflect a mixture of basalt, opal, and porosity (for
example, the 6.87 ct cabochon showed a 4% weight
gain when left in water overnight). The S.G. range
of small pieces of opal without matrix was
2.05–2.15. A spot R.I. of ~1.46 was obtained from
pieces of opal with polished faces, which is consis-
tent with prior tests (Johnson and Koivula, 1996).
The XRD scans for the reddish orange and color-
less opal showed them both to be opal-CT, that is,
having a disordered cristobalite-like structure with
varying degrees of tridymite stacking (Elzea and
Rice, 1996).
Etching of the opal for SEM imaging proved to be
more difficult than expected, implying that etch rates
for both the spheres and their matrix are more uni-
form than those seen in most other opal. We were
Figure 12. This thin section shows details of the opal-filled vesicular cavities within the basalt. At left, in plane-
polarized (normal) light, the opal in the cavities appears colorless. A thin yellowish line (appearing white in hand
specimens) is visible around the opal in each cavity. This unknown fibrous mineral lined the inside of the vesicles
prior to deposition of the opal. The basalt is comprised of abundant well-formed rectangular crystals of white
feldspar together with minor opaque minerals (black grains) in a brown glassy groundmass. At center, the thin sec-
tion of Leopard opal is viewed between crossed polarizers. The elongated feldspar crystals show a preferred orienta-
tion approximately parallel to the lava flow direction, and the opal displays blue and green diffraction colors. In
some cases, the opal diffraction color is consistent across several vesicles, while in others several opal domain
boundaries lie within a single cavity. At right, the homogenous blue color indicates that the opal precipitated as a
single domain of ordered spheres across the large irregularly shaped vesicle. Photomicrographs by R. Coenraads;
height of field of view is 0.9 mm (left), 2.2 mm (center and right).
therefore unable to use the SEM to discern the inter-
nal structure responsible for the opal’s play-of color.
DISCUSSION
Opal-CT is typical of volcanic opal from Mexico
(Smallwood, 2000; Fritsch et al., 2002), and this iden-
tification indicates that the Zimapán opal formed at
reasonably low temperatures, probably between
100°C (Elzea et al., 1994) and 190°C (Rondeau et al.,
2004). According to Elzea et al. (1994), it may have
precipitated originally at even lower temperatures
(~45°C; Rondeau et al., 2004) as opal-A (i.e., with an
amorphous pattern) from silica-rich solutions due to
water interacting with silica-rich volcanic ash and
tuff, and then converted to opal-CT during heating
associated with regional tectonic uplift. There
appear to have been at least two phases of opal depo-
sition, as indicated by the presence of opal of two
distinct bodycolors in the veins. As the orange-red
bodycolor of fire opal is caused by iron-rich nanoin-
clusions (Fritsch et al., 2002), some of the silica-bear-
ing fluids undoubtedly were iron-bearing.
The simultaneous occurrence of a number of fac-
tors, all critical to the formation of opal showing
play-of-color in a vesicular basalt, highlights the rar-
ity of this Mexican Leopard opal and the low likeli-
hood of finding a similar deposit elsewhere:
1. Availability of silica. It is reasonable to assume
that the silica-bearing solutions responsible for
the deposition of the opal in the vesicles and frac-
tures within the basalt flows percolated down-
ward from the immediately overlying felsic tuffs
and breccias.
2. Permeability of the vesicular basalt and porosity
with an advantageous size. Permeability is
essential for the silica-bearing solutions to pene-
trate the basalt and reach most of the open
spaces. The size of the vesicles is also important,
since the abundant, but small, vesicles and their
uniform distribution, are necessary for an appeal-
ing product. Fractures in the basalt have allowed
accumulations of pale blue opal and fire opal
that are also of interest.
3. Environmental factors. These would have favored
the formation of opal instead of cryptocrystalline
silica, and also favored formation of play-of-color
opal over common opal (potch). The silica-bear-
ing waters would need to be introduced over a
sufficiently long period of time and at relatively
low temperatures to lead to the precipitation of
opal-CT (Elzea et al., 1994). We can assume that
the deposit has not been subject to a significant
heating event, which would have led to recrystal-
lization of the opal into a more common cryp-
tocrystalline silica product.
CONCLUSION
Significant potential for Leopard opal exists within
the upper portion (top few meters) of the vesicular
basalt flow, adjacent to the contact with the overly-
ing tuffs and breccias. There is also potential for the
recovery of play-of-color opal and fire opal from
cracks and fractures in the upper surface of the
basalt. Apart from the shallow trenches that may
represent pre-Columbian workings and the small
exploration pits opened up by one of the authors
(ARZ), the Zimapán deposit of Leopard opal has not
been developed and few in the gemological com-
munity are aware of the potential of this material.
At least several hundred meters of contact exposed
on the hillside remain unexplored.
LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006 245
Figure 13. When this
6.87 ct cabochon (left,
under normal lighting) is
exposed to long-wave
UV radiation (right), the
basalt matrix is inert,
while the opal stands
out as bluish white spots
(stronger under long-
wave than short-wave
UV) against the black
background. Photos by
Robert Weldon.
246 LEOPARD OPAL FROM MEXICO GEMS & GEMOLOGY WINTER 2006
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ABOUT THE AUTHORS
Dr. Coenraads (coenraads01@optusnet.com.au) is a con-
sulting geologist, geophysicist, and gemologist based in
Sydney, Australia, as well as a lecturer for the Gemmological
Association of Australia (GAA). Mr. Rosas Zenil is the owner
of Calizas y Carbonatos Rossin S.A. de C.V., a limestone
mine and crushing plant located in Zimapán.
ACKNOWLEDGMENTS: The authors are grateful to con-
sulting geologist Angel Ramón Zuñiga Arista (Lerma,
Mexico), who provided geologic and topographic maps;
Rad Flossman (New South Wales University, Sydney), who
prepared the thin section; Ross Pogson (Australian
Museum, Sydney) for carrying out the XRD scans; and
Anthony Smallwood (University of Technology, Sydney) for
the SEM study. Dr. Edward Littig, Kelly Snedden, and
Charles Rasico (Newman University, Wichita, Kansas) are
thanked for their assistance in obtaining the photo of the
Aztec warrior statue. The authors are also grateful to John
Koivula (AGTA Gemological Testing Center, Carlsbad) for
the photo of the leopard head carving; Dr. Graham Henry
(GAA, Sydney) for assistance with specimen photography;
Kenneth Shannon (Corriente Resources Inc., Vancouver)
for marketing information; and Dr. Emmanuel Fritsch and
the late Dr. Alfred Levinson for their review of this
manuscript. RRC would also like to thank the Cabrera fami-
ly for their warm hospitality and assistance during visits to
Mexico. This study was funded by Worldmin N.L.
www.gia.edu/gemsandgemology
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