MINDAT.ORG 2018 BENEFIT AUCTION Click here to preview now. Bidding starts June 1st
Log InRegister
Home PageAbout MindatThe Mindat ManualHistory of MindatCopyright StatusWho We AreContact UsAdvertise on Mindat
Donate to MindatCorporate SponsorshipSponsor a PageSponsored PagesMindat AdvertisersAdvertise on Mindat
Learning CenterWhat is a mineral?The most common minerals on earthInformation for Educators
Minerals by PropertiesMinerals by ChemistryAdvanced Locality SearchRandom MineralRandom LocalitySearch by minIDLocalities Near MeSearch ArticlesSearch GlossaryMore Search Options
Search For:
Mineral Name:
Locality Name:
The Mindat ManualAdd a New PhotoRate PhotosLocality Edit ReportCoordinate Completion ReportAdd Glossary Item
StatisticsThe ElementsUsersBooks & MagazinesMineral MuseumsMineral Shows & EventsThe Mindat DirectoryDevice Settings
Photo SearchPhoto GalleriesNew Photos TodayNew Photos YesterdayMembers' Photo GalleriesPast Photo of the Day Gallery

Fluorescent Common Opal of St. David, Arizona

Last Updated: 9th Apr 2017

By Rolf Luetcke

Fluorescent Common Opal in Saint David, Arizona
By Rolf Luetcke

The story of the common opal in our area goes back to my wife's past. When I met Mary in 1985 I was aware of the gypsum from the hills of St. David in SE Arizona since I had been collecting it here for a number of years already but I had no idea of the opal that was also from this area. In fact, our first meeting was when I had been corresponding with Mary by letter and phone and needed to come to St. David to collect the gypsum and since she lived there I asked if she was interested in going rock collecting. At the time I lived in Bisbee. She agreed and that was our first meeting. We had a nice day collecting the gypsum near the Apache Powder Plant where she had collected with her family a number of times. We found we had a lot in common and after 32 years have not regretted getting together. We have looked back often and thought that it was kind of an odd first date but for mineral collectors it fit perfectly.
As we collected the gypsum we talked about all kinds of mineral related things and she mentioned that her folks had also collected the common opal in the area since it was fluorescent and her father had been very interested in fluorescent minerals. It was not until we had started a mineral business on the Benson city boundary that we started taking an interest in trying to find a good source of the opal. We have 5 acres of land just outside of the Benson city limits in Cochise County, right along highway 80. When we started our store we could use varieties of minerals to sell and the local opal was a good thing since it was a very nice yellow-green fluorescence under all the wavelengths. Having dogs we often went on dog walks and close to our house is a hill that we later started calling opal mountain. More of a remnant ridge between the major washes of our area than an actual mountain, there was a deposit of the common white opal along parts of the top. The hill was actually a ridge that was a former level of the land before the washes cut the land and left a few of these finger ridges. They were flat on top and from a few feet across to about 50 feet at a few places. The end of the ridge, toward our house stood about 45 feet tall and was at about a 70 degree slope and dropped off sharply on three sides. The end of the little ridge had a small caliche outcrop and we often saw a big red-tailed hawk sitting here observing the country below for rabbits or other prey. We also found pecan shells here, brought by the ravens that raided the pecan groves in St. David and used the open hill with a good view of the surrounding country to break open the pecans. Outcrops like this often had the pecan shells. The finger ridges were remnants of the former land level which had been cut by the numerous outflow washes from the mountains to the west. The whole ridge ran only about a couple hundred yards and on some parts were the layers that had the opal. At the top the ridge was only about 15 feet from the wash on one side and the land became level with the ridge on the other side. There were other of these finger ridges in the area and all seemed to have a bit of opal but the one nearest us seemed to have the most and was closest to our house. Looking at the valley from above one sees multitudes of washes as outflow from the mountains and these finger ridges run all along the valley but we have only extensively looked at the ridges closer to us. Opal mountain is within half a mile walk of our house and when we needed more opal to sell I would take four of the plastic shopping bags they use for the grocery store purchases and go to collect opal. The bags doubled or tripled were light to carry in ones pocket and could be filled with 20 pounds of rock and with a foot long stick through the handle parts made for easy carrying. Without the sticks the thin plastic would wear into ones hands as we carried them. The local yucca fruiting stalks that lay in many places made for perfect handle pieces. Any time I needed opal I went to the opal mountain and picked up what I needed for the store. I did this maybe twice a year. The empty bags also doubled as a watering bowl for the dogs. I always carried a bottle of water and when the dogs got thirsty I made a depression in the dirt or sand and placed a bag across it and filled it with water. This worked perfectly and was not bulky or heavy.
We had many a customer that would ask about where to go collecting local rocks and since we were in the business of selling the material ourselves we often told people to go out to the places they had in the books and then just explore. One older couple came to Arizona every winter and became regulars at the store. They never came without asking about the local opal and they just wanted to go out and pick up a little to say they had collected it. I resisted for a number of years in telling them about opal mountain but in the intervening years I had gone on lots of walks with the dogs and found many other places that had opal and a lot of places with much better opal than our first find so when the couple came in again hoping for information and I was feeling generous I told them how to get to opal mountain. There was a dirt road near us that actually went closer to the hill than walking from our house directly and I told them where to park and how to find the place. I also said it was my find and please not to pick up too much. They again said that it was only for the two of them.
A few months later I needed more opal and took bags and the dogs and walked to opal mountain. When I climbed the little trail I used to get up onto the top I saw a different place. Where before opal lay all over the surface of the hill and I had only to bend down to pick up the nicer pieces of opal, there was not one piece of opal on the whole hill. Even the places where I saw there was opal that went into the ground a bit it had been totally dug out. Believe me I knew who was responsible and I was not a happy person. At first I thought they had been mineral dealers and wanted to collect as much as possible. I did notice that those two never came into my store again. I didn't find out until a number of years later what had happened. I was at the small Sierra Vista mineral show run by the local club in Huachuca City that I found out what had happened. In the inside room of the show they always had a small room set up for fluorescents and I had given a friend a couple of the larger opal pieces for their display and often went into the fluorescent room to see their display and see if they added anything new. As I stood and looked at the display I overheard the man who ran the display talking to a visitor and that is when I found out what happened on opal mountain. It was dark in the room so you couldn't see who was in there but all I needed was to hear the story.
The couple that came in our store and told the story of wanting to collect a bit of a particular mineral just for themselves had actually been looking for places to take the club. I found out that it was an often used ploy by members of mineral clubs and certainly not a nice method but they said they only wanted to actually be able to find something themselves when they were actually scouts for clubs. In this case the man talking to a visitor at the fluorescent room told the person that they did know of a place for the opal but that their club had gone there and cleaned it out. I never said a word although I had felt like it but since I had found many better areas I had never told anyone about I had a bit of a smile that the guy was telling people there was no more of the fluorescent opal since the area had been collected out by them. Sadly it is not a rare situation and over the years we had the store people often came in and asked about collecting places and we gave them the same answer by pointing to the book on collecting in Arizona and that was it. Mary had a different approach with people but she usually left the room when they started with asking for collecting locations. Her response would have been in a very blunt way, tell them to find it exactly the way we did, lots and lots of leg work and looking. People so often want to be led to the great collecting spots by hand so they can find cabinet specimens just lying around to be picked up. Well, even for us those places have never fallen into our laps. With the gypsum collecting we have done that for about 45 years and in our display cabinets are a piece of great gypsum here and a piece there, all found over a long period. I cannot even imagine the actual miles we have walked in exploring the areas around St. David but I bet it is in the hundreds over 45 years. The gypsum and the opal are not like other minerals where they deposited in one area and were mined. These deposited in small pockets where the conditions just happened to be right in one place and then another a distance away. Alluvial deposits tend to be like this. I have heard from friends that the gypsum is found all up and down the San Pedro River valley in many places, not just St. David but I have never checked.
Going on the dog walks to places a bit farther from our house we came across many places that also had the opal and in larger quantities than the small opal mountain. One spot about a mile walk from our house had opal that was very bright under UV. It was another small hill similar to opal mountain but the opal in the wash had come from an exposed layer 2/3 of the way up the steep hill. The hill was only about 35 feet or so high but at one area the layers were right to allow opal formation and it had deposited there at one time. Digging into the hill at the opal layer exposed larger pieces to several pounds of exceptional color under UV light. It seemed that all the opal we actually dug from the ground was surrounded by a layer of white and powdery material, an indicator of opal to be found. It was the best color we had seen in all the dozens of places we had picked up opal. It is a place we dug material when we needed it and never told anyone about it. I have not been in business for a number of years now but was by the hill a couple of years ago and the natural erosion of the hill above where we dug filled in our diggings and unless you knew it was there no opal showed on the surface. The places that were exposed with the white powdery material when it rained washed away the light, powdery material and left the opal behind. It was only in a couple of places where we actually found the white powder still in place, one was while digging and the other in a steep canyon wall.
During the time we needed material for the store Mary remembered a place she had gone to with her parents when they were in the mineral business many years before. It was up a dirt road that went into the hills west of the San Pedro River valley. We knew the area and had even driven the road a number of times in our various trips with the dogs. It showed me that you can drive right by good collecting areas in a car and never know they are there. She said one place we went through a gate was where her folks had walked the fence to where it dropped off into a canyon. She said they used to bring out large pieces of opal and not the normal white but this opal was a bit yellow and a prettier color than the plain white. We drove up to the gate and parked off the road beyond the gate. Mary said she had not been there in many years but said it was where the fence ended. We worked our way cross country through the thorn and got to the end of the fence where the canyon dropped off. I saw opal lying about where the fence ended but below I saw larger pieces lying all over. The slope was steep and dropped about 50 feet to the canyon below. The opal here was on one level about 2/3 of the way up the side. The place was exactly like Mary had remembered and the opal was here in decent quantity. It seems that nobody else had used this area for collecting since they had discovered it so many years ago. The best was to go all the way down to the bottom and pick up the larger pieces that had eroded out of the hill above, saved having to do any digging. We collected there a number of times but in recent years the property was bought up and a gate put in and locked up to entry with no trespassing signs posted on the gate a few miles below. One day we decided to walk into the canyon itself and explore a little. We didn't find any more opal but we did find other deposits of gypsum that differed from the other places we had collected. This canyon also had some of the two million year old mammal fossils that the St. David area is known for. One odd find was a two inch by one inch jagged piece of quarter inch thick steel. It looked like a piece of shrapnel my parents had shown me from bombs in the second world war. Winston was along on this trip and he knew what it was. In 1953 the Apache Powder plant that was about a mile away produced dynamite for the mines in Arizona. At that time they used nitroglycerine for making the dynamite and their one huge storage tank had blown up and killed a number of people and shattered windows all up and down the valley. The piece I had found was a piece of the original tank that had been blasted over a mile from the actual plant. Winston found another piece on that same hike.
My brother in law Winston Macnab told us a story about this road. It was a dirt road that went from St. David up to the north end of the Whetstone Mountains to a canyon that has an old quartz quarry. Now there is a 4 lane divided highway that runs north and south along this section of the Whetstone Mountains but when Winston was a young man this road didn't exist. He remembers using an old pickup truck he had as a young man to drive this road to the mountains to hunt. The one steep spot about a mile below the opal spot was a very steep climb from the valley below to the higher plain that went toward the mountains. The road was so steep at this point that the company that had the quartz quarry and brought out the quartz to Benson and the railhead there for flux in smelting had to build a second road that wound down this very steep part, only a quarter mile long but if two trucks from the mine came to this place at the same time it was impossible to pass so they built a second road. The quartz quarry was long closed but the road was still there. The one route up the steep part had been abandoned after the quarry closed but the remaining part was pretty much a 4 wheel drive part of the road. Winston had driven up to the Quarry one time to go deer hunting and as he came down again his brakes gave out. He said the truck had a really low first gear and he said he made it down all the way back home and even made it down the steep part without losing control. In those days there were no other ways to get down since tow services were not for things like that and you just tried to make it and he did.
The same road to this place also had a side road that went into another canyon a couple of miles away. This was ranch land and the local ranchers ran cattle in the area. The side road went steeply into a little valley with an old cabin near the wash, a windmill and water tank. The old homestead had only partial earthen walls and a collapsed roof but the windmill was new and had a working motor and the large cement water tank on the hill was used to water the cattle on the open range. There were signs on the windmill and tank to keep out and we saw that the windmill had a nice motor that could be run by a portable generator. The water tank had several feet of water with algae and plenty of small fish. The local ranchers stocked the tanks with mosquito eating fish and it was fun to see the tanks with fish in them. I have come across a number of the larger tanks and all had fish. The large water tank sat up on a hill and below by the windmill was a water trough for the cattle and wildlife to use. The upper tank had steep sides and a piece of heavy timber had been placed by the tank to be able to look into it to check the water level. There were well used animal trails coming to this isolated source of water. They were cattle trails but deer, javalina, coyotes and all the other wildlife tracks could be seen in the dust on the trails. Near the tank and windmill was a corral for cattle and one could park by the corral and walk down to the small wash behind the corral and the whole area of a small hill beyond the wash was completely covered by the common opal. There were pieces that were larger than any other opal we had found in the area and Winston and I often went to this place to collect opal. This place seemed not to have been discovered by anyone else and there was no evidence of any other collecting that had been done here.
One such trip to the old homestead we took my old VW bus to collect a load of opal. I had the VW for off road collecting and the gas gage showed half a tank of gas. We collected our opal and started to drive up the very steep road going out of the little valley when the VW stalled. We rolled back down and the car started again but another attempt at the hill had the same results. After a number of checks I took off the gas line from the carburetor and cranked the starter and found there was no gas. The gage had stopped working and stuck on half full. No other thing to do but walk the 5 miles down the wash to the first houses below and have Mary pick us up. I had the two dogs along since they loved outings and always had plenty of water along. It was late afternoon on a summer day and my only worry was that with the long walk the rattlesnakes could be out and with it getting dark we may not be able to see them but all went well. Fortunately it was when I was much younger and in good hiking shape. The next day we drove up, filled the VW with a few gallons of gas and drove home with what we had collected the day before. From then on my off road vehicle ran on the number of miles from the last fill up and I never ran out of gas again. Since it was an old vehicle and didn't need new parts I just wrote the last mileage of gas fill up on the dash when I needed more, giving about a gallon leeway. When we had gotten to the VW that morning to fill up with gas there was a note on the car window. Winston knew the rancher and he was wondering why our vehicle had been parked there. We drove over to his house and explained why we had been over there and he told us that the problems he had were that off road people had been over at his windmill and had stolen the motor off of his well several times and shot the blades of his windmill full of holes. Since it was my vehicle and not Winston's which the man recognized he had taken down our license in case things went missing from his pump. When he found out what we had been over there for things were fine. Since that time the man has passed away and his children sold the property to others and they locked up the land and there is no more access.
One trip to this spot we dug at a large seam of the opal and when we got the piece out we realized we could not carry it back so we drove home since it was not that far and I got my old red wagon where I had replaced the old metal with a wooden box and we drove back. With the help of the wagon we were able to get the boulder of opal to the car and both were able to lift it into the VW. When we got home we weighed it and it turned out to be 175 pounds of opal. The pieces were also very artistic and not just lumps of opal but with holes and various odd shapes. The opal from here was not the top UV color but it was never the less fine under UV.
A hike behind our property on one of the dog walks took us to a steep wash that went to an area we had found the calcite pseudomorphs after gypsum crystals. On the way up the wash another wash came in from the south and on one day we decided to go up this wash. It didn't go up very far, maybe a couple hundred yards until it went up in steep steps of caliche layers in a waterfall area to the higher land above. If one were skilled at climbing one could get up the three layers to the top of the little canyon. In the walls of the canyon below the first layer of caliche were spots of white and also black that looked like some kind of mineralization deposit. They were little pods of bright white powdery material. The black layers I later found were mostly of powdery iron mineralization. Wondering what the white powder was we dug into one. As soon as I dug my rock pick into the material a cloud of dust went everywhere and caused coughing and sneezing. After that we were very careful with the excavation to hold our breath every time we picked out some of the material. At the center of this white powder were nodules of solid opal. To this day I wonder if the opal was altering to the powder or whether the powder was the breeding ground of the opal in the center.
When I got the white powder home I put it under the microscope and realized why it was so allergenic. It was made of tiny splinters of quartz material. The splinters were very thin and this easily floated on the air. Not a good material to ever get in your nose or lungs. On top of the material being in quartz like slivers the opal material in the area was also radioactive. When we first had use of a geiger counter we tried the opal and found it was about three times above the basic ground level of the soils of our area. There is a base reading on most places and the opal was radioactive but only mildly. Now this white powder turned out to be three times more radioactive than the opal itself. The uranium content of the soils here came from outwash from the Whetstone Mountains to the West of us by a few miles where marginal uranium deposits had been found in the 1940's and 50's. We also tested the black material thinking it could also be of uranium origin but it had no reading at all.
The uranium seemed to be concentrating in these pods of white silica material. We don't know the method of deposition here but the soils have layers of caliche that don't allow much moisture to go through. The gypsum of the area and it seems also the opal forms in layers above the caliche or clay which hold water after rains and allow deposition of minerals and crystals.
We have found opal in many places of our area but in recent years the land has been shut off to access and both the area where the 175 pound boulder came out and the fence line place Mary's folks had discovered many years before both are behind locked gates now. I know there are still places we have not been to that also have the opal in quantity but most are only accessible to hiking and not any off road vehicles.
One time a number of years ago a friend called me about the fluorescent opal and asked if I knew where one could get a whole lot of it. I asked him why and he said there was a company in Japan that was looking for a large source of the fluorescent opal to use in dyes they made and it was an ideal material. I told him I knew of a lot of places where small quantities were found but the large amounts he was talking about I had never seen in this area. I could see finding maybe hundreds of pounds but tons was just not in the area here. The other problem was that filing a claim was complex and not something I wanted to even research. The ownership of the land where we had been collecting was not known and although there was private land and public land, we didn't know where any of the boundaries were.
I have used the opal a few times to make cabachons and the material does cut and polish quite well. The opal often has inclusions of iron mineralization and some works up nicely with the inclusions. There are few other minerals I have ever found in association with the opal. Some of the larger pieces I wrote of above had not only the areas of opal but there was alteration taking place as well. The opal had some areas that were seemingly denser and a different color and it looked like they were turning to chalcedony or a quartz instead of opal. Again I don't know if one came first and altered to the other or the other way around. In a very few pockets of this altering opal-quartz I did find tiny clusters of yellow-brown aragonite crystals. The opal doesn't seem to form with other minerals in any of the places we have found except for this one time. None of the opal in our area has ever had a play of colors as in precious opal but we have seen it from white to yellow in base colors and sometimes with inclusions. I have only found one piece that was very translucent in all the years I have collected here, all the other is opaque. That one translucent piece is also the brightest fluorescing piece I have collected here.
Besides the gypsum our area is well known for the opal is also here in good quantity but it has not been as often collected as the gypsum.

Article has been viewed at least 3837 times.


Nice article, Rolf.

I live in northern Nevada and we have opal in the dry washes below the mountains here and there as well. I first became aware of it when I was walking on a stormwater catchment dam near my house. This large construction used rock from a wash nearby, but I never have figured out where. Here and there on the sides of the dam, where the fine-grained material has washed away from the coarser material, one can find small pieces of common opal. It also fluoresces a nice green. It is white, often with a chalky outside rind that is not fluorescent, or only weakly so. I used to take my daughters there to find it. My younger daughter found the largest piece ever but was no larger than a tennis ball. One of the pieces I found was somewhat translucent, and it also had the best fluorescence of any of them. I think the translucence allows the UV rays deeper into the material, and also allows more of the response light back out. We never found any precious opal there. Our few trips have largely exhausted the opal there, but it was fun for the girls to go. One time we came across five or six baby horny lizards that must have just hatched. They were easy to catch, and had blue bellies. The girls enjoyed holding them. They were at most an inch and a half long, and were all within about one square yard. We also found one piece of sandstone with fossil shells in it.

I work in the mining industry, and sometimes have opportunities to see exposures others don't get to see. One such place was the upper benches of an old mine. The material was dry wash with weakly consolidated beds of gravel, caliche, reworked ash tuff, and the like. I immediately noticed opal in the material that had sloughed off the wall above, and since I had rice bags in my truck I was able to collect 35 pounds of it in about 15 minutes, and that was selecting only the best pieces. There were hundreds of pounds of it in varying quality. The largest piece was about 4.5 inches to a side and 1.5 inches thick. The fluorescence was not as good as the material at the catchment dam. However, I could easily see the seam of opal in the wall above. It was up to 2.5 inches thick, and very continuous over about three hundred feet. To the right the layer came down to the bench I was on, but by then the opal was no longer present. The layer ran up and out of the pit to the left, but I never walked up the shoulder of the hill to see what was coming out of the ground there. So I could see the layer from within the pit, but not reach any of the opal in it. They have done more mining since then, and may have wiped out the whole hill. I have not been back in over four years.

The layers enclosing the opal were silty-sandy, reworked volcanic ash, probably a few million years old. It was not clear at all why opal had formed between only those two layers in the layering. The shapes of the pieces were also very odd, with holes and depressions. Very exotic shapes. I have never been able to think of a reason for such shapes. It is strange stuff, and yet not rare, apparently.

I collected several more sacks of that stuff, and sent some to my friend in NC to see if he could find a market for it, since it is much less common over there, but he couldn't find any interest in it. I still have sacks of it lying around but I'm not sure what I'll ever do with it. Maybe I'll take it to the catchment dam and toss it around for others to find. It won't be in-place, but neither is the dam.

I have always wondered why, in all that opal, I have never found even a tiny patch of precious opal. It seems like precious opal deposits often have loads of fluorescent common opal, but the conditions for precious opal must be highly specific within the range of conditions that allow for common opal to form. Another mystery.

Keith Wood
9th Apr 2017 2:50pm
I had collected opal like yours as well, but near Seminoe res Wyoming. It was on field camp. The paleontologist called it chert. I disagreed and knew what this was, called it opal in my reports. When we returned it fluoresced real bright like vaseline glass or something.

Someday i will go get some more. I only collected a handful and sold it.

Matt Neuzil
11th Apr 2017 7:38pm
Thanks Rolf! Nice read.
David K Joyce

David K. Joyce
14th Apr 2017 1:36pm

In order to leave comments to this article, you must be registered
Mineral and/or Locality  
Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2018, except where stated. Mindat.org relies on the contributions of thousands of members and supporters.
Privacy Policy - Terms & Conditions - Contact Us Current server date and time: May 23, 2018 00:40:18
Go to top of page