Amethyst Amygdules at Temporal Gulch, Arizona

Over the past few months, we’ve spent a fair amount of time tromping around Temporal Gulch, near Patagonia, Arizona. This scenic area lies within open cattle range and upper grassland at an elevation of approximately 4,200 feet. We understand that it is Arizona State Trust land, which makes it publicly accessible for recreational use.

Just 15 minutes from Patagonia via a gravel road, the area is easy to reach and makes for a rewarding day of mineral collecting. To get there, drive a couple of miles north out of Patagonia on First Avenue, continuing past the turnoff to the landfill. After the first gate beyond the landfill road, four-wheel-drive vehicles can descend a short distance to a convenient parking area. If you're in a two-wheel-drive vehicle, it’s best to park just inside the gate at the top of the hill and hike down.

From the parking area, follow the path that runs alongside the barbed-wire fence to reach the collecting sites. The area is not only accessible but also offers a pleasant hike and promising mineral finds in a beautiful desert grassland setting.

Basic Geology and Mineralogy

Geologically, Temporal Gulch is relatively simple, consisting primarily of amygdaloidal basalt flows with localized zones of intense silicification. In several areas, these basalt flows host numerous amygdules, vesicles, and geodes that may contain collectible minerals. While many of these cavities are lined with small quartz crystals and later filled with iron oxides, calcium carbonates, or other secondary minerals of limited interest, others are more rewarding.

Some geodes display attractive amethystine quartz and, occasionally, well-formed crystals of baryte. The geodes we’ve encountered range in size from approximately 1 to 10 cm across.

There appear to be multiple zones within Temporal Gulch where amethyst-bearing geodes can be found. We believe we’ve only scratched the surface—more specimens likely remain to be discovered by those willing to explore and keep a careful eye on the ground. Fortunately, it’s difficult to get lost in the area: the eastern side of the gulch is bordered by a now-closed landfill site, which often remains visible and serves as a useful landmark.

Collecting and Preparation

Collecting involves hiking the area and keeping a sharp eye out for quartz crystals and amygdules. When you find one, there’s a good chance more are nearby. In amygdule-rich zones, specimens can often be found on the surface, or you can scratch or dig down to uncover additional geodes.

There’s no reliable way to determine what’s inside a geode without opening it. We typically collect complete nodules larger than 40 mm and bring them home for further examination. Using a splitter, we crack off one end to inspect the interior. If it appears to contain interesting mineralization, it's a keeper.

Most of the geodes are filled with solid calcite and iron oxides that conceal any internal crystals. Occasionally, you may get lucky and expose beautiful amethyst right upon cracking—always a pleasant surprise. However, in many cases, the true character of the specimen isn’t revealed until the calcite is dissolved using acid. For this, we use muriatic (hydrochloric) acid to carefully remove the calcite and expose the underlying minerals.

Above: Abby shows off a nice amethyst-in-calcite crystal that she found at one of the areas.

This locality is especially easy to access and, while not known for rare minerals, it offers an abundance of amethystine quartz crystals. More importantly, it’s a beautiful and enjoyable place to spend the day—whether or not you make a significant find. We’ll definitely be returning.