35 Years. I Still Don’t Know.
Part 1: It shouldn’t reflect green. It does. We’re going to find out why.
I have a garnet in my lab that I cannot explain. I collected it as a teenager. I have been a professional mineralogist for over two decades. I still cannot explain it.
The crystal faces are catching the light and throwing it back metallic green — not an internal color, not a gemstone’s green, but a cold, iridescent metallic green, the kind you associate with the wing of a beetle. I turn it in my hand. The color moves. It is not a reflection of the sky.
The Place
The Aquarius Mountains in western Arizona are not a place that announces itself. From a distance they look like every other range in the Mojave transition zone — brown, dry, relentless. Up close they are steeper than they appear, the rhyolite cliffs dropping in vertical columns above a scrub desert that goes on until it doesn’t. You find garnets here by walking the talus slopes below the outcrops, eyes down, until the light catches something that doesn’t belong to the dirt.
My family has been coming here for decades. We are not the only ones. The black andradite garnets of the Aquarius Mountains are known to collectors — not famous, not rare enough to command serious money, but known. You pick them out of the rhyolite matrix or find them loose in the soil, perfectly formed dodecahedra, black and mirror-bright, sitting in their rust-orange weathering pockets like they’ve been waiting. Most of the time, that’s exactly what you find. Black. Bright. Expected.
The first time I found one that reflected metallic green, I was probably fifteen years old.
My father found it first, or something like it. I don’t remember the exact moment — I remember the object. A garnet that shouldn’t look the way it looked. The crystal faces were catching the desert light and throwing it back metallic green. Not dark green. Not olive. A cold, iridescent metallic green, the kind of color you associate with the wing of a beetle, not with a silicate mineral sitting in volcanic rock in the middle of Arizona. I turned it in my hand. The color moved with the light. It was not a reflection of the sky.
We had no framework for it. We had a conclusion instead: this was a new mineral species. What else could it be? Garnets are black or red or orange or green in the gemstone sense — transparent, colored by chemistry, passive. They do not reflect metallic green. They do not behave like metal. Whatever this was, it was not in the field guides we had brought.
We collected every one we could find. They were rare even then — most of the garnets on those slopes are the ordinary black kind, and the metallic ones appeared only occasionally, scattered without obvious pattern among their unremarkable neighbors. We carried them home convinced we were carrying something that had never been described.
What the Worn Edges Showed
Years passed. I kept collecting. I kept coming back to the locality, or sending family members who still make the trip. More specimens accumulated.
And then something happened that should have been a disappointment: I noticed the worn edges — and some areas where the metallic surface looked like it had peeled away.
Some of the metallic garnets, on their most exposed faces, had worn through. The metallic green was gone on those edges. And underneath — black. Ordinary, mirror-bright, andradite-black. The same mineral underneath, with something thin and green on top.
This was not a new mineral species. It was a coating.
That should have closed the question. Instead it opened it into something stranger. If the metallic green is a coating, then what is it? How did it get there? Why are some garnets coated and others — sitting right beside them in the same pocket, in the same rhyolite, under the same desert sun — not coated at all? Was the coating deposited when the garnets formed, twenty million years ago? Or later, by something else entirely?
I became a mineralogist. I still had no answer.
What I Now Know About the Garnets Themselves
The Aquarius Mountains sit in Mohave County, Arizona, on the boundary between two of the West’s great geological provinces — the Colorado Plateau rising to the northeast, the Basin and Range dropping away to the southwest. The mountains themselves are the transition, the place where the high, ancient platform of the plateau gives way to the stretched and faulted terrain of the desert below.
The rhyolitic volcanic field here is roughly 24 to 20 million years old — Miocene, late enough that the landscape we recognize was beginning to take shape, early enough that the specific volcanic episode responsible for these garnets is essentially undescribed in the published literature.
That last part is worth pausing on. The garnets are known. Collectors have been finding them here for decades. But the published record on this locality is thin and confused — sources disagree on which garnet species is actually present, and the formation mechanism has not been properly documented.
What I believe I am looking at is andradite — calcium iron garnet, Ca₃Fe₂Si₃O₁₂ — formed by vapor-phase crystallization directly in the rhyolite. This is an unusual formation pathway. Most andradite forms in skarns, where magma contacts limestone, or in hydrothermal systems where hot fluids move through rock and deposit minerals in fractures. Vapor-phase crystallization in rhyolite is a different process entirely: minerals growing directly from volcanic gases, without a liquid intermediary, in cavities within the cooling lava.
If that formation mechanism is correct, these garnets grew from steam.
The metallic green coating formed on crystals that had already solidified from vapor. Whatever deposited that coating was a later event — a fluid, an alteration phase, a chemical reaction between the garnet surface and the environment around it after the volcanic episode had ended.
That sequence matters. The garnet is one record. The coating is a second record, written on top of the first, in a language I haven’t translated yet.
The Question, Properly Stated
I still have the original specimens from that first family trip. They are sitting in my lab at the Natural History Museum of Los Angeles County, next to newer ones collected by family members who still make the trip to the Aquarius Mountains. Thirty-five years of the same question, accumulated in a tray.
Here is what I know: the metallic green is a coating. It is nanometers thin in places — thin enough that the worn edges of some crystals have lost it entirely. It is not uniformly distributed across all the garnets at this locality, even among garnets sitting in the same pocket. It has not been published. Nobody, as far as I can determine, has identified it in the literature.
Here is what I don’t know: what it is. What phase it is. What chemistry. Whether it formed at the same time as the garnet or later. What chemical conditions were required to deposit it. Why some garnets have it and others don’t.
The lab now has instruments that can ask the question properly. What those instruments are, and what it takes to use them on a coating that may be only nanometers thick, is the subject of the next post.
This is the first post in an open investigation. I don't have the answers yet. Neither does the literature. The next post goes into the lab.
I am the Curator of Mineral Sciences at the Natural History Museum of Los Angeles County, former scientist at NASA's Jet Propulsion Laboratory, and adjunct professor at USC. I write Pocketful of Χtals because mineralogy is stranger and more alive than most people have been told.
All photos by Stan Celestian
Some (well, one out of the 7 I have) of the vapor-phase garnets from the Thomas Range in Utah also have an iridescent sheen. Mine is more bluish than yours. Looking forward to your next chapter!
This is so exciting.