Coyamito agate is a nodular banded agate found about thirty miles north of the Laguna agate deposits in Chihuahua, Mexico. It is found in multiple deposits, most of which are located on the Coyamito Ranch. Forty years ago, agate was collected and mined on the Coyamito Ranch, La Gregoria Ranch, El Agata Ranch and probably the El Sueco area and sold as Coyamito Agate. This resulted in a variety of agate referred to as Coyamito. However, most of the strongly colored agates come from two areas on the Coyamito Ranch.
In many parts of Mexico each square kilometer is given a name. This name defines the area. The two areas on the Coyamito Ranch that have agate deposits are called Los Alamos and La Sonoreña.
Los Alamos has only two deposits that I know of. They are about 500 yards apart and are located about two miles west of the Coyamito Ranch house. The original claim on these deposits was called La Fortuna. The claim is now called Los Alamos after the name of the area. One of these deposits is a long trench dug by hand in the early 1960s on the side of a hill. This old digging is pictured in the book, The Agates of Northern Mexico, by Brad Cross, page 45. According to some accounts, this was one of the original areas mined on the Coyamito Ranch. The other deposit at Los Alamos is a relatively flat area close to an arroyo, which had the reputation of producing agates with yellow and purple colors. It was this area close to the arroyo that I worked in 2003 and was profiled/described by Brad Cross in the March 2004 issue of Rock and Gem Magazine.
La Sonoreña is located about two miles north of the Coyamito Ranch house. It covers a canyon with an east flowing drainage and contains many small deposits with extensive old diggings (see map).
Near the mouth of this canyon is as old corral built entirely of stones piled on top of each other. The stones are piled 6 ft high in a perfect rectangle about 50 yds. long and 30 yds. wide. It was built well over 100 years ago. I camped here during my mining operation in April of 2004.
Coyamito agate nodules have the reputation of being small and colorful. Coyamito Agate mined in 2004Of all the banded agates found in Chihuahua, Coyamito is considered the closest relative to the Laguna agate. Like Laguna agate, Coyamito is characterized by colorful tight banding. Most Coyamito nodules are hollow and the color strength is in the outside banding. Other features include pseudomorphic agate, which is agate replacement of aragonite (see essay), and tube formations. Agate pseudomorphs are usually rare, but are common in Coyamito agate.
The agates are formed in gas bubbles in an andesite rock called the Rancho El Agate Andesite (Cross pg. 35 ). This host rock covers a large area of the Gallego Mountains but only a small portion of it has cavities.
Part of my agreement with ranch owner, Marin Carillo, was to work at least 5 of the deposits at La Sonoreña to determine the feasibility of continued mining. I numbered these areas as I worked them and they are indicated on the map below.
Area 1 was closest to the camp and consists of numerous small hand diggings up a shallow slope and several extensive diggings in a line on the hillside. The miners of long ago were following float agate up the hill until they found the host rock and then they worked the host rock until they got muck bound. I tried several places here. Some had no agate and others had agate periodically in the rock. The host rock was full of holes…just not full of agates! There are some very fine agates from this location with fine contrasting banding. There are also some unusual very dark or black banded agates. All the nodules are typical tear drop shape and are of fairly consistent size. I found no large agates here.
While our mining operation produced many small nodules, we also discovered many large nodules. This was a surprise, but probably a result of working with a machine rather than by hand, and working in new areas.
About fifty yards to the west, at the same elevation, a wash enters the arroyo from the south. This is area 2 on the map. The wash drops off steeply into the arroyo and this area had been worked very hard in the past. Andesite rocks ranging in weight from 20 to 100 pounds and full of holes were worked from both sides and the middle of the wash.
I tried this area for three days and only found three agates that were of exceptional quality. There were all very large with yellow and violet banding and a little moss agate in the nodule. They were similar to the Agua Nueva agate nodules that I mined 6 or 7 miles to the south in 2000. There was very little agate found at this location for the amount of work necessary to produce it, and I do not have a good idea of why the miners of long ago worked so hard here.
Further up and across the other side of the arroyo is area three. From the extent of the old diggings it appeared that this deposit might be a little larger. Whether or not the deposit extends further than the others I did not have time to determine, but the agates from this area are generally larger. One or two pound agates were common and several were found over 5 pounds. However, the general quality of the agates was mediocre and I did not find any exceptional pieces here.
Areas one, two, and three are all at the same elevation. Area four is located close to a saddle between the tall mountain and a large hill several hundred feet above the others. I did not know about the existence of this deposit until after I was already working at area one. After I inspected the area, I decided to build a road up to the deposit and try working it.
The nodules from area four are rounder in shape than the nodules from most of the other areas, and red is the predominant color. Many nodules contain a contrasting white or pink band just under the surface. The general quality of the agate from here is high with 50% of the production graded as number one material. The host rock is very hard and the agates are not as concentrated in the host rock as they are in other locations. Many small red nodules came from here but I did find large ones also. A beautiful red and yellow banded agate with a large tube formation is pictured in the 2005 Calendar of Fine Agates and Jaspers, produced by The Gem Shop, Inc.
I spent considerable time walking around Sonoreña inspecting old workings contemplating what areas might be worth mining. There are about 20 such workings in the canyon where I was camped, one in the canyon to the north, and another out on the flat in front of the canyon to the north. In the third week of my work I decided to test a few spots past area 3 that I had looked at the year before. These were small deposits and turned out to be unproductive. Past these deposits on the edge of the adjoining arroyo I found another old excavation. This area was hidden by a low hill and I had not seen it before. It is quite large and, because of the way it was worked, it was difficult to notice until you were right on top of it. This turned out to be the area most suitable for mining and is area five on the map.
For some reason this area was not worked as hard years ago. All sizes and qualities of nodules are found here including a good percentage of clear colorless agates. I worked area five the last three days of my time at La Sonoreña and had my best day of quality production for the whole operation the very last day. That same day I loaded the excavator on a truck to be returned to Chihuahua City.