The Mexican Opal
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.
- INTRODUCTION
- PHYSICAL AND CHEMICAL CHARACTERISTICS
- TYPES OF OPAL
- TERMS FOR COLOUR IN OPAL
- COMPOSITION OF OPAL
- OCCURRENCE
- THE OLDEST MINES
- CANTERA DE OPALO
- COMMENTS
- CHARACTERISTICS OF OPALS FROM QUERETARO
- INCLUSIONS
- MARKETS
- BIBLIOGRAPHY
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.
This Special Issue of Gemmology Canada is published for students of the Canadian Institute of Gemmology and others interested in gemmology. The copy-right for any articles remains with the author. Opinions reflected in this article are not necessarily those of the editor. For further information write to C.I.G., P.O. Box 57010, Vancouver, B.C. V5K 5G6 or phone/fax (604) 530-8569 or e_mail webmaster@cigem.ca
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