Mexican Geodes: Coconut, Trancas and Tabasco Types

Almost every geode sold as Mexican belongs to one of three groups, and nearly all of them come from Chihuahua in the north of the country.

Las Choyas coconut geodes are the round, fist-sized ones with grey rinds and crystal centres. Trancas geodes are smaller, thin-shelled and sometimes fluorescent. Tabasco geodes are the tiny split pairs sold in display boxes.

They formed in volcanic rock rather than limestone, which is why their shells are different from Midwestern American geodes: the cavities started as gas bubbles in rhyolitic ash flows, and mineral-rich water later grew crystals into them.

These are the ones sold in the trade as Mexican coconuts, and Rock & Gem magazine has documented the deposit in detail.

• Where and how. They come from one small area north and east of Chihuahua City. Miners drill and hand-finish shafts about three feet in diameter through rhyolitic ash tuff, following a geode-bearing zone that tips downward at about ten degrees. The working depth is now over 125 feet below the surface.
• Size. Shop pieces usually measure 2.5 to 3.5 inches, ranging up to about six.
• The odds. About 80% of the mined geodes turn out to be solid. The solid ones go to the lapidary trade for cabochons and decorative pieces such as bookends, because solid agate cuts and polishes well.
• Inside. Eighteen minerals have been identified, with quartz and calcite the most common. Calcite appears in several habits and colours, alongside siderite, quartz varieties and manganese oxides.

A coconut geode is a reasonable first geode to open with children, because the shell is thick enough to survive handling and thin enough to open without a saw.

Trancas geodes were found in Chihuahua in 1971 and are usually just two to four inches across, with thin shells in grey, brown, cream or peach.

Inside, the usual finds are clear and drusy quartz, chalcedony, calcite and aragonite, with occasional quartz scepters and curved, finger-like quartz growths that make some cavities look like miniature caves. Most are hollow with crystal-lined cavities, though some have solid agate centres closer to a thunderegg.

The feature collectors want them for is fluorescence: many glow green under shortwave ultraviolet light because of trace uranium in the minerals. The amount involved is small, but shortwave ultraviolet lamps themselves are a hazard to eyes and skin, so use eye protection made for UV work, never look into the lamp, and keep it away from other people while it is on.

On the collector market they are not rare or high-end material; well-formed drusy examples are the ones people compete for.

Tabasco geodes are among the smallest geodes sold anywhere — around half an inch across, split cleanly down the middle and sold as matched pairs, usually in a small display box. The interiors show fine drusy crystal layers, often reddish.

They are a curiosity rather than a specimen, and they are priced accordingly: one specimen dealer listed a split pair at US$12 in September 2026, marked as unavailable at the time.

One honest caution about the name. Dealers do not agree on where they come from, with listings variously giving a town called Tabasco and the volcanic field of Chihuahua. Tabasco here is a trade name, so treat any specific locality claim as something to ask about rather than assume.

What to ask before you pay:

• Were these sorted, and how? Given that about 80% of Las Choyas geodes come out solid, sorting by weight is the difference between a good afternoon and a bucket of rock.
• Whole or pre-cut? Pre-cut pairs have already been opened and matched; whole ones are the lottery.
• Any treatment? Bright dyed colours are common in agate products generally, and disclosure is required in the United States.

When you open one, Trancas shells are thin and forgiving while coconut geodes are thicker. Score a line around the middle and use even pressure from a chain pipe cutter rather than a bare hammer, which is more likely to shatter a geode than halve it. Wear eye protection whatever method you use, and put the geode in an old sock if children are helping.