Peridot in Colorado abstract from Rocky Mountain Gemstone Symposium, Colorado School of Mines, Golden, Colorado, September 7, 8, 9, & 10th, 2002
Peridot in Colorado
By John A. Rhoads
D&J Rare Gems, Ltd.
A small but significant deposit of gem grade peridot occurs within the San Isabel National Forest on the border of Fremont and Park counties in the vicinity of the junction of these two counties with Chaffee County. The map coordinates are N 38 degrees 41.50 minutes, W 105 degrees 51.60 minutes at an elevation of approximately 9,580 feet. The location can be found on the Cameron Mountain 15 quadrangle and 7.5 Gribbles Park USGS topographic maps. Access to the area can be obtained by taking Chaffee County Road 175 just north of Salida, Colorado, into Fremont County where it becomes Fremont County Rd 2 and turning left at the Everett Cattle Camp on the road to Herring Park and Cable Gulch. A rough dirt road that branches off from this road about two miles from the junction leads to the top of the mesa. The flat top mesa with the peridot bearing basalt
can be seen in the distance to the north approximately 2 miles away from the junction. The two mesas encompass an area of approximately 3 square miles in a very irregular shape. This areas has been known to produce gem grade peridot for many years although it has only been since the early 1990s that it became common knowledge to collectors.
The area appears to be covered by at least three separate episodes of basalt extrusion that occurred during the Tertiary Period. Capping the mesas is a reddish, highly gaseous basalt, known as scoria, that is barren of any included minerals. In some areas the cavities left by the gases are so numerous that even the largest boulders can be lifted with ease. Another episode of deposition of red basalt contains mostly black hornblende xenocrysts. Some of these inclusions are very rounded indicating some reabsorption during the migration of the magma. The peridot occurs in a gray, very solid basalt almost completely lacking in gas cavities. This basalt usually occurs beneath the other two episodes of deposition. This basalt is very tough and efforts to remove any of the peridot it contains usually results in fracturing of the peridot long before the basalt breaks. Underlying the basalt is a gray limestone, possibly the Leadville Formation of Pennsylvanian age.
Small gem grade pieces of peridot are found weathering out of the basalt that caps two small mesas in this area. The upper flat areas of the mesa are generally barren of peridot with most occurring on the slopes below the top appearing to weather out several feet beneath the upper most layer. The best collecting areas appear to occur where the slope is gentle just below the weathering layer allowing for the mineral grains to accumulate rather than disperse down a steeper slope. Over a dozen different locations on the slopes surrounding the mesas have been found to contain gem grade peridot although some are more productive than others.
Peridot is also found within the basalt in knots (peridotite xenoliths) up to 5 centimeter in diameter although these knots are highly fractured and do not produce single rough pieces of comparable size. Smaller, solid grains of peridot under a carat in size are predominate in some basalts while the highly fractured knots occur in others.
A second location, known as the Lone Pine area which is a small basalt knoll located approximately 2 miles south of the two mesas and is easily located and identified about a mile east of the junction of Fremont County Rd 2 and the Herring Park / Cable Gulch Road on Fremont County Rd 2 by the lone pine tree that grows on the northwestern edge of the deposit. This deposit contains a mix of both loose grains of peridot that have weathered out of the basalt as well as those frozen in the host rock.
The color of the peridot cut from the rough from this area is a very fine, medium, lime green. The color is fairly consistent throughout the deposit. Finished gems are very bright and popular among collectors for their fine color and luster.
Inclusions in the peridot are rare. Two gems cut from rough found approximately a mile apart contain during the 2002 collecting season contain similar, high relief, rounded crystals that so far have not been examined for identification.
Sizes of the rough and cut gems from these locations are small compared to peridot from other locations throughout the world. Most of the rough grains of peridot are well under a carat in size. Grains over a carat are fairly common with those over 2 carats being exceptional and any over three carats a major find. The largest rough piece known weighed 8.17 carats, however, the shape was very irregular and the gem cut from this piece of rough weighed only 1.54 carats. It is currently in the Denver Museum of Nature and Science collection. The largest recorded cut peridot from these locations is a 2.87 carat gem Robert Spomer cut for a client. A cushion cut gem weighing 2.47 cts was cut from a piece of rough weighing 5.44 carats by John Rhoads for a client. This gem was particularly clean of excellent quality. Another gem of 2.08 carats was cut from rough found in the Fall of 2001 by John Rhoads and is currently in the DMNS collection. Numerous gems between 1 and 2 carats in size have been cut by John Rhoads with several of them currently in the DMNS collection. Rumors of recent finds of rough as large as 14 carats have gone unsubstantiated.
Other minerals that are found in association with the peridot are black cleavage fragments of amphibole group minerals. Gem grade fragments of colorless quartz is found with the loose peridot. Some light purple quartz (amethyst) has been cut with a knot of this material discovered intact in September of 2001. Also a dark green mineral of gem grade has been found in association with the peridot that tests within the indices of enstatite although verification of this identification has not been established at the time of writing this report. (The gem cut from this dark green minerals has since been identified by GIA as "enstatite". December 2002)
Gem grade peridot has been reported from several other areas in the vicinity of this deposit but as of this writing none of them have been confirmed.
Current status of the area is pretty much open to collecting. During the mid-1990s Robert Spomer and several associates filed the Green Beebee claim on one of the more productive areas to keep it open to collecting. A few years later a group from Canon City, Colorado, have filed claims to the north of the mesa and are currently active according to one of the owners. Please observe their claim markers if in the area. These areas have since been heavily collected by rockhounds. The best time for collecting is immediately after a storm or following the melting of snow that covers the area during the winter. The area is usually accessible to collecting from late April until November. Caution should be taken if collecting during hunting season as this is a very popular area for deer, antelope, elk, and bears.
If planning on visiting this area to collect respect any current claims. Do not bury but carry out all trash. Remain on current roads with vehicles as flora in this area is very fragile and takes years to recover from even a single off road venture. Should you visit the area and find an exceptional piece of rough that results in a superior finished gem both Jack Murphy and Pete Modreski as well as myself would like to hear about it.
John A. Rhoads
D&J Rare Gems, Ltd.
1 719 530 0628
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