Mineral of the Month

MINERAL OF THE MONTH

Spring 2009

Coyamito Agate from Chihuahua, Mexico

The mineral of the month is the Coyamito Agate from northern Mexico. It is mined on a ranch located around 40 miles from the world famous Laguna Agate location, which is around 75 miles south of El Paso, Texas. The ranch is in the middle of the Chihuahuan desert at an elevation of just under 5,000 feet. For the past 50 or 60 years, the Coyamito has been one of the most sought after Mexican agates. They display a wide array of colors with fine banding, and the most unusual pseudomorphs of any agate on earth. Both of the specimens featured in the photos below are agates that formed when silicon dioxide replaced aragonite crystals.

When a highway was built through this desert during the 1940s, rockhounds began venturing into the area. Mineral claims were made in the late 1940s and prospectors have been searching for these valued agates ever since. By the 1960s, most of the agates were already extracted. More searching resumed in the early 1990s, when two people died while using explosives. Rockhounding on the ranch was prohibited until the last few years, when special permission was given to a few well-known agate collectors. These prospectors had to quarry directly into 38 million year old andesite, and carefully, but with much difficulty, remove the nodules from the very hard matrix.

The specimen pictured on the two left photos is a sagenite-like pseudomorphs that has botryoidal formation on the top and back. The specimen on the two right photos is a pseudomorph Coyamito agate with a drusy quartz layer on top. The appendages most likely formed when agate replaced a needle-like mineral, such as goethite. You can see in the far right picture that in cross-section, the agate bands have an eye-like stalactite formation. I purchased this specimen already cut into the three sections. I am selling this unusual group as a set. If you are interested, please send me an email to karen @ agatelady.com.

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