by Carmen Ramirez, A.G. (C.I.G.), Queretaro,
Mexico
Editor: This article was translated into English by Carmen. Great job, Carmen! The picture of the carved Mexican opal is part
of the MEXICAN OPAL
SAFARI slide presentation by J. Wolf Kuehn. The greek "opallious" literally means to see a change of colour. Opals have been in the world since the beginning of humanity; there is
no doubt that the world of opal is ancient. Pliny describes the word of opal
about 55 A.D. in his book natural history.
The word "opal" comes from the Latin "opalus", from the greek "opallious";
the greek word comes from two roots, the first relates to seeing, as in our
words opaque and optical, the second meaning other. It has also been suggested that the word opalus could have come from
the name of the roman god of fertility "Opus", the wife of Saturn, her
celebration was called Opalia. Alternatively the word is supposed to be derived
from the Sanskrit "upala", a word used to mean precious stone. History claims that Cleopatra of Egypt had a round black opal set in
a necklace and when Marco Antonio saw the opal he admired the beauty of the
queen and the beauty of the stone. There are opal mines in different places of the world: for example Germany,
France, Spain, New Zealand, Australia, England and Mexico. The characteristics
of each opal are different, each country produces a different type of opal.
PHYSICAL AND CHEMICAL CHARACTERISTICS FORMULA: SiO2 n H2O water= 1-21% in opal. CRYSTALLOGRAPHY: Amorphous, no form, because opal is an aggregate composed
of tiny spherical particles of silica. COLOUR: The opal comes in different colours for example white, yellow,
orange, colourless, red, yellowish-brown, greenish-blue, gray, black, violet,
with various shades.
LUSTER: Vitreous to waxy. HARDNESS: 5.5-6.5 Mohs scale
DENSITY: 1.99-2.25 (orange red variety), -2. 00 (black and white opal),
2.10
(green opal), 2.0 (fire opal). CLEAVAGE: None, Fracture conchoidal. OPTICS: Single refractive, Isotropic, N=1. 44 and 1. 47 fire opal 1.
40 mexican opal as low as 1. 37. DISPERSION: Very low. LUMINESCENCE: Green fluorescence in opal often due to included minerals,
much opal fluoresces strong white in SW, LW. In opal from Queretaro, Mexico
the reaction with SW: Dull white, phosphorescent and with LW will be bright
blue, phosphorescence. There are many types of opal. SILICEOUS SINTER, GEYSERITE: that is a massive, glassy opal that forms
around hot spring and geysers. DIATOMACEOUS EARTH TRIPOLI: fine grained, powdery masses of opal of the
siliceous remains of microscopic marine animals called diatoms. PSEUDOMORPHOUS: opal may in percolating through the ground, replace wood,
bone and shells. HYALITE: transparent, colourless, or white to gray generally no gem
significance. COMMON OPAL: opaque or glassy opal, in a wide range or colours sometimes
with a waxy luster, often fluorescent. WATER OPAL: transparent, colourless opal that may have fire in it. FIRE OPAL: transparent to translucent red or orange which, may not have
fire in it, the term fire opal refers to a body colour not to a play of colour.
PRECIOUS OPAL: opal of any colour with fire. WHITE OPAL: white body-colour opal usually with play of colour. GRAY OPAL: light to dark body colour with play of colour superimposed.
BLACK OPAL: black body colour with fire. body colour also very dark,
bluish, greenish, or brownish. MILK OPAL: milk white translucent, also yellowish or greenish in colour.
CRYSTAL OPAL: water opal or milk opal, generally rich in fire, transparent
to translucent in transmitted light, colours seen by reflected light. CONTRA-LUZ OPAL: very rare type usually from Mexico with colour play in
both transmitted and reflected light. HYDROPHANE: light coloured, opaque becomes iridescent and transparent
when soaked in water. JASPER OPAL: reddish-brown opal, opaque, resembles jasper. CACHALONG: porcelain ferrous, often bluish-white very porous. PRASE OPAL: translucent or opaque green opal a common opal resembling
prase. MOSS OPAL: white to brownish opaque opal that contains dendritic inclusions.
MENILITE: opaque gray to brown opal with a concetionary structure. TABASHEER: opaline silica occurring in the joints of bamboo. GIRASOL: opal that is almost transparent and has a billowy light-effect
within it, resembling moonstone. CRYSOCOLLA IN OPAL: blue material, with finely disseminated chrysocolla
that gives the colour. LIVER OPAL: term used sometimes to describe brown opal. RESIN OPAL: yellowish brown common opal with a waxy luster. FIRE: the term fire refers to the magnificent play of colour displayed
by opal which is due to light diffraction from neatly stacked layers of the
microscopic spheres of which opal is composed. Common opal is a jumble of
spheres of random sizes, but in precious opal the spheres are the same size
and they are layered in neat rows the particular colour depends on the size
of the spheres and the angle of viewing. ONYX OPAL AND AGATE OPAL: alternative layers of precious and common opal.
In cats-eye opal the colour play is concentrated in the form of and eye or
band. Matrix opal consist of specks of precious opal in a rock matrix usually
sandstone, matrix opal may also be layers or stringers of opal in a rock
matrix. FLAME OPAL: sweeping reddish streaks, bands move across the gem,
resembling flickering flames. FLASH OPAL: as the gem is moved back and forth flashes of colour appear
and disappear at various spots. HARLEQUIN OPAL: the colour display is in the form of angular patches,
all in contact with each other like mosaic. PINFIRE OPAL: the colour is in the form of tiny dots, set close together.
PEACOCK OPAL: many colours appear in the same gem resembling the display
of the tail of the male peacock. IRIDESCENCE: this is the play of rainbow-coloured light caused by extremely
thin layers of regular structure beneath the surface of the gem stone, like
a thin film of oil on water, these layers interference with the reflected
light rays reinforcing some colours and canceling others. The effect is seen at its best in precious opal, opal play of colours
is caused by millions of submicroscopic spheres of cristobalite, silica gel,
which make up the bulk of the stone. These spheres are all the same size in precious opal and are arranged
in orderly rows and columns because of their small size and the symmetry
of their arrangement, the colour reflected light by a combination of interference
and diffraction effects. This later effect is produced when white light is
split up into spectral colours by being passed through a narrow aperture.
The colour produced by precious opal depends somewhat on the angle of
viewing, but mainly on the size of the spheres. Opal is made up of microscopic spheres of Silica quartz without the quartz
crystal structure, ranging in size from 200 to 300 nanometers in diameter
at one time play of colour was the thought to be due to the interference
of light as it passed trough the microscopically thin layers of spheres.
Interference is the phenomenon that creates the iridescence. Play of
colour is caused by a combination of diffraction and interference; it works
like this.
Waves of light passing the edge of and object are bent slightly as they
pass, a characteristic called diffraction. If two edges are close enough
to form a narrow opening the waves fan out, bending around both edges, the
waves of the fan begin to overlap like the waves of the ocean when they pass
through their pilings of a pier. The overlapping causes interference-certain
spectral colours are intensified and others are cancelled out. The silica spheres in opal are close-packed, arranged like oranges in
a crate, the spaces between them act like narrow openings through which light
is diffracted, the diffracted light gives birth to the intense spectral colours
of opal. The colours visible in an opal depends on the size of the spheres
and the voids between them. The opals occur in sedimentary rocks or where low temperatures solutions
bearing silica can percolate through rocks.
Mexican opal occurs in siliceous volcanic lavas, in cavities and in many
localities. Yellow and red fire opal comes from a trachyte porphyry at Zimapan
in Hidalgo.
Hyalite and precious opal that is completely transparent, colourless
and rich in fire occurs at San Luis Potosi, Chihuahua, Queretaro. Fine mexican
opal is very rare in very large sizes, over 50 carats, but is among the most
beautiful.
Opal is also formed as a sedimentary deposit and was washed out of silica
bearing rocks to solidify in fissures and crevices and thin veins of silica
gel material.
A Mexican author says that the opal mines are in between the mountains
of igneous rocks, where you can find obsidian, volcanic glass, the ground
in those places is very dry and the vegetation is mostly captus. The colour
of the rocks varies from one region to another, it goes from deep red to
gray sometimes almost white. The best opals of Mexico come from regions of
mountains of volcanic lava. The principal regions of opal mines in the state
of Queretaro are:
* Tequisquiapan
* San Juan del Rio
* Colon
In an area of about 150 square kilometres are found some of the best
producing opal mines such as LA HACIENDA LA ESPERANZA I894 LA LAJA Y GALERAS belonging to Colon and Toliman
LA HACIENDA LA LLAVE FUENTEZUELAS LA FUENTE LA TORTUGA HACIENDA GRANDE SAN NICOLAS in Tequisquiapan and San Juan del Rio DEL PERDON I893 LA CARBONERA I883 LA TRINIDAD LA MEJOR DEL CIERVO SANTILLAN SAN FRANCISCO CHARCOS in Cadereyta. The oldest mine in Queretaro is Santa MARIA del IRIS this mine was opened
around 1870 and has been reopened 28 times. At the moment there are about
a 100 mines in the regions but most of them are closed. The mines that had
produced most opal are 1. -SANTA MARIA DEL IRIS 2. -CARBONERA 3. -TRINIDAD 4. -PERDON 5. -JURADA The best quality of opals come from Santa Maria del Iris and the mines
of La Hacienda la Esperanza. the second place belongs to the mines of
Fuentezuelas and the third place belongs to the mines of Carbonera and La
trinidad. There is a mine called Napoleon 1896 that produces opals in quality very
similar from the mines of Queensland, this opals are very thin and milky.
1. SANTA MARIA DEL IRIS Hacienda la ESPERANZA 1870 2. -La de los Dolores La Carbonera, Hacienda la Llave, El Garambuyo,
Hacienda Fuentezuelas 1883 3. -La Trinidad, Hacienda San Nicolas el grande 1885 4. -San Miguel, La Simpatica, Hacienda la Esperanza 1888 5. -El Perdon, Hacienda los Charcos 1893 6. -La Esperanza, Hacienda la llave 1894 7. -La Mejor, El Divino Redentor, El Refugio, La Jurada 1895 8. -Los Charcos 1897 9. -La Minita 1898 In Tequisquiapan where there are some opal mines the region has also
soft thermal waters deposits that contain magnesium, calcium and other
substances.
In other region of Queretaro where some opal mines are found there are
also a lot of chalcedony rocks. After the opals of Queretaro the second production in the country of
importance are the opal mines of Jalico in Magdalena andTequila the name
of the famous mines are LA UNICA, SAN SIMON, SAN ANDRES. This mines produce
the typical arlequin opal of Jalisco. There are other regions in the country that also produce opal like Guerrero,
that produces a opaque opal similar to the opal from Australia, some of these
opals are carefully treated with heat to improve the colours. There are also
opal mines in Morelos, Durengo, Chihuahua, Baja California, Gajuato, Puebla,
Michoacan, Estado de Mexico. In Magdalena Jalisco facet grade fire opal is called opalo cristalino
o vidrio rojo meaning crystalline opal or red glass, while near the city
of Queretaro the same material is known as agate rojo, or red agate. Much
of Mexico's precious opal has a translucent body colour in the same warm
range of yellow, orange or red. Other opals have a hazy looking with strongly
scatters of blue light, giving the stone a translucent blue character while
still displaying a brilliant play of colour. Some fire opals are clear enough for facet and some shows the prismatic
display of precious opal, most of the opals are a bit of hazy. THE CONTRA LUZ OPAL meaning against the light is another kind of Mexican
opal when this opal is viewed by reflected light, this opal looks like
colourless, but when the same stone is look against the light, this stone
shows a brilliant play of colours like the finest precious opal. HUESO O YESO another kind of opal is this one that has a white body colour
similar to the opals from Australia, after cutting the stone, the stone might
be declare to be hueso or yeso, meaning bone and gypsum. QUEMADOS meaning burned, most of the yeso opals are wrap in foil paper
and then buries it in soil or sawdust in a small pot, then the pot is heated
enough to create a partial burning of organic materials this process changes
the body colour of the cut stone from white to dark, grey or black, without
affecting the opalescent play of colour. Mexican opal is most of the time
found filling small gas cavities in rhyolite or similar hard volcanic rocks.
Miners must guess where the most productive rock lies in the mine and blast
and hammer it into small chunks, to find nice stones. Many of the opal filled cavities are so small and irregular in shape
that they cannot be cut into cabochons this stone is left in the matrix,
and then the cabochon with opal and matrix together is cut. With this material
some people carve animals and faces as these stones are less expensive than
the opal stone without matrix. Opals has water in its composition, the exact amount in any particular
stone depends on the nature and the environment of the stone, when the opal
is mining it environment changes, a small percent of opal will crack within
a few days after being mined. Rough material that has been stored in a dry
climate for a month or two is not likely to crack during cutting.
Some Mexican opals are transparent only when recently soaked in water,
with in a few hours of drying, they become hazy and the haziness will continue
to develop until the stones have become completely opaque, showing a pink
orange colour resembling coral, often this stones will completely regain
their clarity after being soaked in water for an hour or two. CHARACTERISTICS OF OPALS FROM QUERETARO The opals are often found filling former gas cavities in a reddish to
pinkish rhyolite lava flow, the colour of the opals vary from white to a
reddish orange to colourless, some of them are transparent and other translucent.
The most common type of inclusion are black needle crystals. Many of
these crystal inclusions had at least partially altered to goethite and many
had almost botryoidal coating of hyalite opal, which was likely deposited
at an earlier time than the precious opal.
The dark needlelike inclusions appear like distorted hexagons in cross
section, many are brownish red in colour, probably because they have altered
to goethite, several of the inclusions have a coating of hyalite opal.
Occasionally the center of a opal is cloudy or even hollow, this inclusion
is called huevo meaning egg. Most Mexican people will not buy an opal because they believe it is a
gemstone of bad luck. Most of the opals are exported to the United States,
Canada, Germany, Spain and other countries of Europe, but the finest quality
of opal are sold to Japan and Hong Kong. EPIFANIA DEL OPALO, by PASCUAL CUTLILLAS, MEXICO 1965 ESTUDIO SOBRE EL OPALO, PRIMERA Y SEGUNDA EDICION by MANUEL MUNOZ LEDO
Y MENA, MEXICO 1929 COLOR ENCYCLOPEDIA OF GEMSTONES by JOEL E. AREM, PhD, F. G. A. GEMMOLOGY, by PETER READ OPAL IDENTIFICATION AND VALUE, by PAUL B. DOWNING, PhD LAPIDARY JOURNAL,VOL 48 NO 3 JUNE 1994. This article was submitted March 30, 1995 in fullfillment of the "Accredited
Gemmologist (C.I.G.)" diploma requirements. The Mexican Opal
This Special Issue of Gemmology Canada is published for students
of the Canadian Institute of Gemmology and others interested in gemmology.
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