The GemHunter

Professor Hausel's Guide to Finding Gemstones, Diamonds, GoldRocks & Minerals 

OPAL & AGATE 

"Surely Love is a wonderful thing. It is more precious than emeralds, and dearer than fine opals. Pearls and pomegranates cannot buy it, nor is it set forth in the marketplace. It may not be purchased of the merchants, for can it be weighed out in the balance for gold" 

- Oscar Wilde


Opals Everywhere! I couldn't believe my eyes. Many, more than a hundred thousand carats, just sitting in the road cuts of an active oil field south of the town of Riverton, Wyoming. How did everyone miss these sitting next to a highway and within an oilfield. There were even boulders of opal sitting on the side of the oilfield service roads and many embedded in the road?

Opals everywhere! All white cobbles and boulders in this photo are opals! 

Within this field, I found beautiful agate, jasper, tons and tons of common opal, tons of fire opal and a few specimens of precious opal all in an area of a few square miles. What lies beneath the surface? What's across the highway to the west, there needs to be much more prospecting and research here. I suspect, if the university or state ever invests in serious research, they will find an unbelievable resource here.

In a reclaimed trench dug t lay a pipeline that runs through the oil field continuing to the south, I came across considerable opal, varieties of agate including the source rock for the famous Sweetwater agates. The late Dr. Dave Love of the USGS reported in his classic professional paper on the Granite Mountains that the source of the Sweetwater agates was unknown. And here is was - right in front of me. 

There must be hundreds of millions of carats of potential gemstones and lapidary material. After I reported the discovery to the press, the BLM had a hissy fit! Even so, it would take decades (if ever) before anyone could come up with a strategy to explore, market, mine and sell this material. The biggest danger is the BLM carrying out the wishes of some of its employees and withdrawing the land from public access. But they have little to worry about. From my experience, putting anything into production requires a very, unique person who knows how to get things done - someone who is possessed! So, personally, I think the BLM is safe.

I began reconnaissance at Cedar Rim (a.k.a. Cedar Ridge) in the Sand Draw oil field of central Wyoming. Based on a clue from a local rock hound from Riverton, it was apparent geologists & prospectors overlooked this deposit. I did a literature search and found that the US Geological Survey had identified opal in the area in the 1930s and 1940s, but paid no attention to the gem, other than to mention it in passing.


 Location map of the Cedar Rim opal field, central Wyoming.

 

But, it is apparent one of the larger opal fields in North America lies along State Highway-135 between Sweetwater Station & Riverton. Graded roads leading to the field, exposed thousands of cobbles & boulders of opal in road cuts. The opal had been overlooked! I kid you not, I found boulders of opal in these road cuts that included some of more than a few hundred pounds - all untouched!


The Cedar Rim deposit, based on size & extent, is one of the largest in North America. As with any colored gem, success requires exploration, marketing, understanding the geology, & cooperation from government regulators, and then a lot of luck. However, as soon as the discovery was announced, regulators began to mark their territory.


The BLM claimed that protection of an endangered flower was paramount even though the special flower turned out to not be indigenous to the area & could only be found where cattle dropped their waste in exposed dirt in road material. It would make every bureaucrat proud (and many were standing knee deep in cow waste). I even had a BLM manager call me demanding to know the location of the deposit so they could withdraw the area before the location of the deposit was even announced. I refused to cooperate. I knew if I told him, the deposit would have ended up in the hands of some manager's cousin, nephew, or uncle.  Even with bureaucrats trying to keep people out of the area, mining claims were staked following my announcement to the press. But no need to worry, most people had no clue as to what they were doing - such as a prospector from northern Colorado who called me from the field right after he staked dozens of mining claims in the wrong township. He was concerned that he could not find any opal - but he was actually 6 miles too far to the east. 


The geology of Colorado-Montana-Wyoming is favorable for discovery of opal deposits. During the geological past, silica-rich volcanic ash ejected from the Yellowstone caldera, blanketed the region several times providing an excellent source for soluble silica necessary to produce opal and agate. Unfortunately, precious opal from Cedar Mountain was rare - but when found it occurred as secondary vein deposits mixed in common opal. 


Opal (SiO2.nH2O), in general, is a precious to semi-precious stone classified as an amorphous mineraloid with 6 to 10% water. A mineraloid is a mineral-like substance that does not yield an exact chemical formula; and like volcanic glass, shows no sign of crystallinity. Opal has a hardness of 5.5 to 6.5 and is fragile to relatively durable semi-precious gemstone. In general, the higher the water content, the less stable the opal.


Three general categories of opal include: (1) common, (2) fire, and (3) precious. Precious opal is the most valuable. Precious black opal is considered to be the most valuable of precious opals by gemologists, because of an internal color play enhanced in a dark matrix. Precious white opal is considered to be less valuable - its internal color play is less distinctive.

 

Fire opal, which may or may not have color play, can be translucent to transparent red, orange-red, orange and/or yellow. If transparent to translucent, fire opal may be faceted: Opaque to translucent fire opal is usually cut into cabochons. Translucent to opaque milky white common opal may contain streaks of blue, red, brown, or yellow, and is also cut into cabochons. Hyalite, a colorless to transparent opal found as globules that resemble drops of water, is most often is cut into cabochons. All varieties have been observed at Cedar Rim either as massive material, or in trace amounts. Cedar Rim is extensive, covering hundreds of acres of land, thus the potential for significant discoveries of high-quality precious opal must be considered, especially since little surface and no subsurface exploration has occurred.

 

Most primary opal deposits occur in sedimentary and volcanic (rhyolitic) rock. It is less common in basalt and metamorphic rock. The majority of the world’s precious opal is mined in Australia where Cretaceous marine sedimentary rock of the Great Artesian Basin of New South Wales, Queensland, and South Australia play host to the opal. There are many opal fields in eastern Australia: probably the most famous is Coober Pedy and Lightning Ridge with underground homes and businesses dug in the opal fields.

 

Opal has low specific gravity, conchoidal fracture, and tends to craze (lose water and fracture). As a result, placer opal is unheard of. Opal preservation is unfavorable where surface weathering has been intense over long periods of time. Alluvial opal deposits are rare and restricted in size and extent.


Exposure to dry environments and heat causes opal to lose water,  resulting in opaque, chalky-white, fractured crusts and masses of caliche replacing opal. Opal cannot survive deep burial nor structural adjustments (movement along faults). Because of limitations in durability, opal, whether sedimentary- or volcanic-hosted, is geologically young.   


The Cedar Rim field consists of vast amounts of white to light-blue opaque common opal, with lesser amounts of translucent to opaque yellow, yellow-orange to orange fire opal and some clear, transparent, hayalite. Only trace precious opal has been identified in samples. In that all varieties of opal have been identified, the potential for discovery of valuable seams of opal must be considered. I identified opal within 12 sections (each section consists of one square mile). Large amounts of agate were also identified including the source beds of the popular Sweetwater dendritic agate. In places opal beds are a few feet to >50 feet thick. Since there has been no subsurface exploration, the true thickness remains unknown.

 

Cedar Rim occurs along Cedar Rim Draw near the NW margin of Beaver Rim. Beaver Rim is a topographic ridge that marks the northwestern edge of the Granite Mountains. The nearest towns are Riverton (25 miles to the northwest), Lander further west, and Jeffrey City to the southeast. The deposit is located on the US Geological Survey Lander 1:100,000 topographic sheet and on the Rattlesnake Hills 1:100,000 sheet.

 

Opal & chalcedony were observed replacing tuffaceous limestone at the top of the Oligocene sediments that cap Beaver Rim as well as in buttes to the south. In places, limestone has layers of massive white chalcedony and opal nodules enclosed in calcareous crusts. The presence of cylindrical pipes of silica cutting through some limy layers is interesting.

 

The source of the silica for the opal and chalcedony is interpreted to have come from underlying (silica-rich) volcanic ash beds. Silica likely mobilized in percolating water which surfaced in springs. The opal and chalcedony occur in the Wagon Bed Formation (now known as the Wiggins Formation), the White River Formation, and the Split Rock Formation, suggesting that the opal is widespread and potentially thick.

 

Numerous chert nodules and silicified zones were found in the White River and Split Rock Formations. Locally, opal and yellowish-brown to light olive gray chert occur as masses up to 3 feet in diameter Wagon Bed Formation mudstone in the vicinity of Wagon Bed Spring and northeastward to the Rogers Mountain Anticline. Irregular opal and chert masses up to 15 feet long are found in the Kirby Draw syncline (runs northwest from NE section 31, T33N, R94W to section 14, T33N, R95W).

 

At Green Cove (section 35, T31N, R96W) the uppermost 20 feet of the lower part of the Wagon Bed Formation has altered yellowish- to light-gray, distinctly bedded tuff with abundant siliceous nodules. These are accompanied by 6- to 12-inch thick chert beds with quartz, dolomite and opal. Within this area, sandy limestone lenses up to 5-feet thick have been partly replaced by irregular fibrous chalcedonic chert and massive gray opal with irregular tubes and pores: many of which are filled with calcareous montmorillonite clay.


Irregular domal structures occur that are several feet in diameter. These are formed of sand adhering to opaline skeletal structures that resembles tuffa or algal mats in the Split Rock Formation. These lie in well-sorted calcareous sandstone southeast of Devils Gap in section 5, T30N, R95W. It was noted that thin beds of chert, irregular concretions of opaline silica, and fibrous siliceous aggregates were found along Beaver Rim in the uppermost part of the Split Rock Formation. These are in 2- to 6-inch thick, light-gray, limestone interbedded with thin calcareous tuffaceous sandstone.

 

The geology of Wyoming, in particular, is favorable for opal due to the abundance of silica-rich volcanic ash from past Yellowstone and Absaroka eruptions. In addition to sedimentary-hosted opal at Cedar Rim, precious opal was described in volcanic rocks in the Absaroka Mountains in northwestern Wyoming (J.D. Love, personal communication, 1989) and in the Yellowstone caldera.

 

Opal samples collected from the Cedar Rim field were found in White River (Oligocene) and Split Rock Formation (Miocene) marlstone, limestone, claystone, siltstone, sandstone, conglomerate and boulder facies. Opal was found in sections 25, 26, 35 & 36, T32N, R95W, sections 31 & 32, T32N, R94W, sections 5, 6, 7 & 8, T31N, R94W, and sections 1 & 12, T31N, R 95W. These include giant opals of 25,850 carats (11.4 lbs), 57,100 carats (25.18 lbs) and 77,100 carats (34 lbs). The known opals range from small cobble size nodules to large boulders encased in caliche. Several varieties of opal and agate were identified:

 

(1) Opaque milky white, tawny to translucent common opal with localized fracture fillings of transparent clear opal. Some have light blue opal with black dendritic inclusions. Some are perfectly transparent. Many are fractured but include large consolidated unfractured pieces of several carats.

 

(2) Translucent light-blue opal enclosed by milky opal which in turn is enclosed by narrow perfectly transparent and banded opal crusts that exhibit a pleasant color play (bands of blue-yellow-violet red) in natural light. Some are enclosed in a thin rim of tan to pink quartz. With further exploration, it is likely that more precious opal will be found.

 

(3) Opal with milky quartz breccia & light gray to light blue translucent to transparent opal clasts & veins set in black opal to black chalcedony matrix. The black opal rarely exhibits color play.

 

(4) Gray black to black translucent opal and agate.  Some material was collected in place in section 7, T31N, R94W.


(5) At one location in section 25, T32N, R94W, a hill was discovered capped by fractured, vari-colored fire opal. This opal forms replacements and fracture-fillings in silicified arkose (sandstone). The material is translucent to opaque yellow, orange and red similar to Mexican fire opal.

 

Other opal fields in the West are found in Nevada, such as the Royal PeacockRainbow Ridge, BonanzaDominion and Virgin Valley deposits. The Spencer Idaho opals are also well known, and some opals are reported in southern California as well as in Arizona and Utah. 

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