A guide to mexican opals

As a radiologist, writing a review on mexican opals was perhaps the last thing I expected to be doing, but having fallen in love with this most beautiful and diverse of gemstones, and given the paucity of information available about this amazing gem on the internet, I have set about acquiring a collection of these stones on okayand have recently visited a mexican opal mine. (see user ymgep's listingsfor details of this amazing trip). I thought I might share some of the knowledge I have acquired to help other users understand this amazing gemstone.
This guide may be of particular interest to those familiar with Australian opal who want to find out more about Mexican opal.
About opals
Most people are more familiar with australian opal than the mexican variety. Opals consist of veins of silica which are found in the host matrix rock. In the case of australian opal veins of opal are often found in ironstone which may be polished along with the opal to produce boulder or koroit opal. Mexican cantera opal, which is the equivalent of boulder opal, is found in pale rhyolite matrix which is softer than the opal itself. The play of colour seen in all types of opal depends on the arrangements of the microscopic spheres of opal silica which diffract the incident light. If the arrangement of these spheres is haphazard there is no play of colour and the opal is referred to as potch opal. A regular arrangement of the spheres is required to produce a play of colour. The size of the spheres determines the colour, with smaller spheres producing the red hue, and larger spheres the purple colour. The colourplay in Mexican opals is often strongly dependent on the intensity of the light in which the stone is viewed, themore valuable stones have colourplay in dim light and the colourplay often takes the form of a rolling flash.
Mexican opal mines exist all over Mexico, many are near the town of Magdalena, where several Mexican opals are sold. One of the largest opal mines is the San Martin mine which produces almost all of the types of Mexican opal described below, many newer mines are appearing all the time and there is much unexplored territory in Mexico which may yet yield opals.
Types of Mexican opal - not an exhaustive list
The terminology is confusing as the same terms cannot be applied to mexican opal as australian opal.
Mexican Fire opal

This term often produces some confusion. Mexican fire opals are clear crystal opalsprized for their base colour, which in the most sought after of stones is either deep orange or red. These stones do not have any play of colour, (in contrast to australian fire opal, the term here is used to describe opals whose play of colour is predominantlyof red colour).
These stones are often found near the surface in an opal mine.
Flash opal

These stones are crystal opals with colourplay. I have seen this group of opals referred to as water opals (there is play of fire in these stones but the base colour is clear), or as crystal opals (although the fire opals also fit into this description). The base colour of these stones may vary and many of the particularly prized stones are clear or orange in base colour. The intensity of the colourplay determines the value, as well as the size of the stone.
These stones are also foundnear the surface of an opal mine, and are usually free of rhyolite matrix.
The physics of the colourplay is explained above. Purple colourplay instonesis the rarest and these gemand the highestprices (in contrast to Australian opals in which red colourplay is most prized).
There is a variant of this form of opal which looks much like Australian opal and looks very attractive, from the La Nayarita mine in North Mexico.
Cantera opal

These stones are geparable to Australian boulder opal and its origins are similar. These are found as small pockets or seams/veins of opal associated with rhyolite matrix, the host rock, which is softer than the opal. The word cantera refers to the host matrix. Like boulder opal and koroit opal, these stones were not initially as prized as the pure cuts but cutters and buyers of both koroit/boulder and cantera opal came to appreciate the attractiveness of the matrix cut and polished with the opal. Mexican opal often requires a dark background to bring out the colour play and the cantera although in some cases quite light, provides a natural backing to the opal, call it a natural doublet-like effect if you will. The most stunning cantera opals have a pool or "eye" of central opal within the matrix.
These stones are found deeper in the mine. If the cantera or matrix is cut away gepletely the stone will appear like a flash opal. These opals are difficult to mine as the rhyolite cantera can be very hard, and they are found by laboriously breaking the cantera by hand and looking for veins of opal.
Contraluz opal

This refers to a property of both mexican and australian opals where when illuminated from behind, the stones develop a brilliant colour play. This effect is rare and it will require the breakage of several tonnes of matrix to get a few of these stones. Flash opals can demonstrate this contraluz effect.
Mexican black opal
These opals are extremely rare and are seen as an extremely small precentage of mexican opal. They are extremely valuable.
Jelly opal
As with Australian opal, this refers to a cloudy translucent stone, with or without colourplay.
Carbonera opal
The opal from the Carbonera mine is similar to Andamooka matrix opal and Honduran black opal, and has small flashes of opal within a basalt type dark matrix.
Pink opal
From the Rositas mine, these opals are pink and show no colourplay. They are particularly prized by the French.
Synthetic Cantera Opal

Man made cantera opals are starting to appear on the market. The matrix is man made and is resin based. These stones can be quite attractive but the cantera does not feel the same as that of authetic cantera opals.
The Market
Mexican opals are particularly prized by the Japanese and Germans to whom the stones of the highest quality are exported. The French are particularly keen on pink opal. Some people remain suspicious of opal, it is considered by some to be unlucky and others to be lucky. It was thought that the the diamond industry was responsible for the labelling of opal as unluckyas itfelt threatened by the emergence of the opal as the most brilliant gemstone showing all colours of the rainbow. This is thought to have stuck with various cultures.
Valuation
Similar variables are of importance as in Australian opal, and more detailed literature is available elsewhere: the important variables include:
Intensity of colour play (the most valuable gems show bright colourplay in even dim light)
Spectrum of colour play - as described above purple colour play is most rare and prized
Pattern of colour play - special patterns such as harlequin and pinfire patterns also exist in Mexican opal and are prized.
Size of the stone - of course in gebination with other factors, a 100 Ct potch opal is not worth much!!!
Cracking and crazing may reduce the value of a stone dramatically. Some types of Mexican opal are unstable so the length of time the stone has been kept may be important when buying your stone - ask your seller!
Rarity is obviously also very important, mexican black opal is very rare and the contraluz crystal opals are also scarce.
Darkness of the base colour is not an issue in Mexican opal, unlike Australian Opal, this is probably because most types of mexican opal are crystal opal. There is no grading of this, in contrast with Australian.
Cut (see below)
Types of cut
Cabochon cut
All of the above varieties can be cut in this manner (see pictures above)
Faceted stones
Fire and flash opals can be faceted, these forms are prized for jewellery (see pictures above)
Carved stones

Carving anunusually shaped stone can greatly increase its yield. Carved opals are often called freeforms. Cabochons can be referred to as freeforms too, these are stones not cut into a regular shape.
Buying Mexican Opal on okay- tips
Many of the same tips apply topurchasing other gemstones too:
Mexican opal is very difficult to photograph particularly as the play of colour is very dependent on the lighting conditions. Read the description carefully in terms of the lighting, howwas the picture taken? Most Mexican opals are photographed in artificial light with a digital camera. Shining the light through the stonecan enhance its appearance.
Read the listing carefully. Some undesirables might be in very small print - doublet is often put in small letters at the bottom of the listing in Australian opal. The stone may be greatly magnified and look great on the photo but when it arrives it's tiny! Look at the carat weight it's not always in a prominent place.
Sometimes it is difficult to tell what the quality will be until you have bought a few items off a seller.I buymost ofmy items off about half a dozen good sellers from whom the quality is always high. I started by buying a few gems off them and buying in higher quantities when I knew what I was going to get and got to know the seller. Good sellers are very keen on repeat custom and will gemunicate attentively with you and even provide freebies.
Feedback is obviously very important and lots is written about this elsewhere. My tips are read it all: Some buyers willleave quite negative gements under the guise ofpositive feedback to avoid damaging their own buying profile.
A feedback score several multiples below the total number of items sold is very good and means that people are repeat buying. For example if a seller has a score of 500 and has sold 3000 items, each customer has bought six items on average - a happy bunch of customers! I think personally that this is even more helpful than the percentage positive.
Use the guide above for valuation of your stone before bidding. A large stone is not necessarily a good one. Cracking and crazing reduces the value of a stone markedly and look for this in the photograph as it is often evident. Many cantera opals do have natural fracture lines and are still very attractive, though. Sometimes you won't know the true colour intensity till your package arrives - so if it is a new seller, try a few items, if you like what you got, buy more next time.
Some real bargains can be had in the 99 cent listings and as you gain more experience you will be able to spot bargains! There are stones worth thousands of dollars with a 99c start, I have bought some myself! The buzz of snaring a bargain keeps us geing back! I think it is risky buying any very expensive gem without some previous experience of the seller - I've been ripped off before (not in the Mexican opal market!).
It may beworthasking how long the gem has been stored for - if it has been stable and not cracked for several months, it is unlikely tocrack or craze subsequently. Some sellers guarantee their stones.
Back to the stones........
Doublets and Triplets
The same terminology here as for Australian opals. Doublets are opals with dark backing material (which may or may not be opal) attached to bring out the colourplay. Triplets are precious opal sandwiched between a dark backing material behind and clear material at the front to protect the stone.
Doublets are generally worth more than triplets and both are worth far less than a geparable solid stone.
The Mine Trip
I have just returned from an amazing trip offered by ym gepany (user=ymgep) to visit an opal mine in the Guadalajara area, and learn how to cut stones. It has been immensely educational and we were extremely well looked after by our extremely kind host. I would highly regemend this trip to anyone who has any interest in Mexican opal.
Ymgep is my personal favourite seller and offers all of the gems described above.
He has opened a new site called mundogem in which there are many more stones.
Please check the box if you liked the guide. Many thanks!!