Geology of Gemstone Deposits

Large chlorite pseudomorph after garnet found in the Sierra Madre Mountains, Wyoming. The chlorite slowly replaced the garnet and accepted the garnet's crystal form. 

Much of Wyoming is underlain by Archean cratonic basement rocks and cratonized Proterozoic rocks that provide favorable geological environments for a variety of gemstones – notably diamond, iolite, ruby, sapphire, garnet, kyanite, andalusite, sillimanite, labradorite, jewelry grade gold, platinum and palladium nuggets, emerald, aquamarine, helidor, tourmaline, spinel, clinozoisite, zoisite, apatite, jasper, specularite, etc. Thick Phanerozoic sedimentary rock successions with lesser Tertiary volcanic rock cover large portions of the basement terrain. Some of these Phanerozoic rocks provide favorable hosts for other gemstones including opal, placer diamond, placer gold, placer platinum, placer ruby, jasper, agate, emerald, varisite, etc.

Using traditional exploration and prospecting methods, dozens of gem and precious metal deposits were discovered over the past 3 decades, including major discoveries and geological and mineralogical evidence for significant undiscovered deposits. Major swarms of mantle-derived kimberlite, lamproite, and lamprophyre, many of which have proven to be diamondiferous, also host colored gemstones including pyrope garnet (Cape Ruby), spessartine garnet, almandine garnet, chromian diopside (Cape Emerald) and chromian enstatite. One lamproite also yielded peridot.

Favorable conditions for crystallization of metamorphogenic gemstones during regional amphibolite-grade metamorphism occurred during the Precambrian. In this terrain, metapelite in the central Laramie Range hosts kyanite, sillimanite and andalusite. These three minerals provide evidence of favorable pressures and temperatures needed for crystallization of aluminous gemstones including ruby, sapphire and kyanite. Cordierite (iolite) another aluminum-rich gemstone, formed during a later thermal event. This later event was responsible for deposition of world-class iolite (Water Sapphire) gemstone deposits.

While searching for gold, I came along
this giant jasper deposit.

Evidence for undiscovered gemstone deposits is predicted based on mineralogical anomalies detected during various research projects from 1977 until 2005. These include ruby, sapphire, gold and aquamarine found in stream sediment samples as well as favorable geological terrains that remain unexplored. Other anomalies include pyrope garnet (several with G10 geochemistry), picroilmenite, and some chromian diopside that provide evidence for hundreds of undiscovered diamond deposits. Elsewhere, detrital diamonds reported by various prospectors provide direct evidence for undiscovered diamond deposits. Other geological and mineralogical evidence suggest the presence of additional undiscovered opal, cordierite (iolite) and kyanite deposits. Wyoming could potentially become a major source for gemstones including diamond, gold, platinum, palladium, Cape ruby, Cape emerald, iolite and opal.

Giant chrome diopside gem in kimberlite from Colorado.

OPAL

Resistant, opal, boulder adjacent to rock hammer in Tertiary
volcaniclastics, with a seam of resistant opal lying beneath the
 boulder and hammer, Cedar Rim, Wyoming.

A rockhound from Riverton, Wyoming, mentioned opal was reported south of town near Cedar Rim at one of my rock hound club talks. When I returned to Laramie, I began searching old US Geological Survey reports on the stratigraphy of the area and found, in passing, mention of opalized rock in some of the stratigraphic columns - so, I headed to the area not knowing what I might find, but the reports left me with the impression that these were just trace amounts. So, when I arrived at Cedar Rim, I was absolutely amazed at the extent of the opal. The great majority of the material I classified as common opal, but also found an entire hillside with colorful red-orange, orange to yellow Mexican opal, and Mexican opal breccia, and traces of precious opal, along with dendritic agate (Sweetwater agate) and a host of colored agates. Based on the presence of the precious opal, I suspect some seams might be found at depth. After spending a few days in the opal field, I later found additional US Geological Survey reports describing some opalized tuffaceous sediments located to the west and east of Highway 135 suggesting that there is likely more opal to the west of the Cedar Rim deposit.

Specimen of 'Candy' opal from Cedar Rim - a mix of white
common opal with colored 'Mexican' opal.

The Cedar Rim deposit lies east of highway 135 and is cut by oil field roads. A few old geological reports from 50 t0 80 years ago mention opal in passing, so I was surprised to find opal scattered over 14 mi2, opal masses containing 80,000-carats along the edge of the oil field roads, and common, fire & precious opal and scattered Sweetwater agate, along with some of the nicest decorative stone in Wyoming. All occurring in Tertiary-age volcaniclastic sedimentary rocks that had a notable

contribution of volcanic ash erupted from Yellowstone in the past.

Cedar Rim boulder opal. All of the boulders and
cobbles that stand out as resistant rock, is typically
filled with opal.

This gave me a clue - nearly all of Wyoming was blanketed by volcanic ash (as was Nebraska and South Dakota). Guess what? There are other opal fields waiting to be discovered. So I found millions of carats of common & fire opal & traces of precious opal (including black opal) that suggest as soon as someone digs, valuable veins of precious opal will be found as depth!

The waxy material in the rock matrix is common opal. 

IOLITE (WATER SAPPHIRE)

A flawless iolite gemstone

What the heck is iolite? Geologists typically describe this mineral as cordierite, while gemologists typically refer to it as iolite or water sapphire. Guys don't have a clue to what this beautiful mineral is, but many women either have a ring with some iolite, or they've seen in on TV. It is every bit as attractive as Tanzanite, but at about 1/10th to 1/20th the price. Why? Because Tazanite was marketed by a retail gemstone company, and iolite has not been marketed.

I discovered iolite (water sapphire) at Palmer Canyon in 1995. The iolite occurred with gem-quality kyanite in mica schist & gneiss adjacent to vermiculite that had enough silica to form these gems. I couldn't believe the amount, size & beauty of the stones. The single, largest, iolite gemstone on earth was found at this time -the baseball sized Palmer Canyon Blue Star- a 1,720 ct flawless gem.

First group of iolites gems cut from
Palmer Canyon rough. The bed of
material that the iolites sit in has tiny
white fibers that unfortunately sit
in front of the iolites at the top. But
these iolites were flawless.

So I went looking in similar terrains & predicted in a book I would find iolite at Grizzly Creek to the south. In 2005, I discovered iolite at Grizzly Creek & recovered the largest iolite gem (24,150 carats) on earth (the football sized Grizzly Creek Blue Giant), but this was dwarfed by giant, VW bug-sized masses in the outcrop I left behind because I didn't have tools necessary to recover such large gems of 1 to 5 million + carats. Based on geology, I predicted several other iolite deposits would be found in central Laramie Range & Copper Mountain, but most remain unexplored. Over the years I tried to get some investors interested in forming a company to take advantage of these giant deposits - explore for more, mine them, cut the gems, design jewelry and market them, but no one wanted to take on this task, so they all essentially sit there collecting dust and eroding away.

But nothing could match the potential discovery further to the south. Some geologists had

explored this region for aluminum & magnesium & during the mid 20th century and they reported one deposit that had been trenched and mapped contained 500,000 tons of cordierite (when gem-quality, cordierite is known as iolite). I began looking at this deposit & found flawless gem-quality ioilte along the edge of the deposit. All of the material reported by theses earlier geologists likely was part of the largest gemstone deposit found in history. But they were so focused on the use of the material for magnesium and aluminum, they overlooked the gem potential. Besides, in the mid-20th century, there was not much of a market for iolite gemstones.

Iolite in outcrop at Grizzly Creek, Wyoming. Note the rusty matrix to the
right of the iolite. This is limonite replacing the iolite. Iolite has considerable
iron and will 'rust' under the right conditions.

But let's look at this deposit as a potential gem deposit. It is unexplored for gems except along the one margin where everything I examined was of the highest quality gem material. The rest of this deposit needs to be examined. Why?  One ton of material contains 4.5 million carats! Multiply that by 500,000 tons and this could potentially have more than 2 trillion carats just on the surface and no one knows how deep it goes. If it is just 100 feet deep (it likely is a few thousand or more feet deep), it would dramatically increase the amount of recoverable gemstones. Iolite sells for only about $15 to $150 per carat, but if marketed like Tanzanite was marketed, one could increase the value of this (and other deposits in this region) to the most valuable mineral deposits on earth! But to develop this will take a lot of money and support.

The Palmer Canyon blue Giant sits on cover of
ICMJ Prospecting & Mining Journal. At 1,750
carats, this was the largest iolite gem reported
in the literature until I found much larger
stones at Grizzly Creek, Wyoming.

So what did I get for finding two of the largest colored gemstone deposits on earth? Well, like the song goes, the director got the mine and I got the shaft. But am I bitter? Yes!

Largest iolite in world at 24,150 carats from Grizzly Creek, Wyoming. Much larger stones were left in outcrop.

GEMSTONES & GEOLOGY

A good understanding of geology, persistence, & luck can lead to incredible treasures. In 1977, I started work as a research geologist at the Wyoming Geological Survey in Laramie. Next to nothing was known about gemstones in the state, but over the years, I tracked down many mineral deposits & found some of the largest gemstone deposits ever found on earth. Anyone can do it. 

My story is found in many publications and blogspots, the most recent available at Amazon, the Wyoming Geological Survey publications office and the University of Wyoming bookstore in Laramie, as well as at your local bookstore.

Some of the many gemstones discovered in Wyoming since 1977.