Boron Open Pit, Boron, Kern County, USA — a major borate locality producing colemanite, ulexite, borax and kernite; prized for color crystals and rare borates.
Key facts
The Boron open pit is one of the great mineral localities of the American desert: an active, world-scale borate mine cut into the Kramer borate deposit in the western Mojave Desert, just north of the town of Boron. For collectors, its importance is twofold. It is an economic landmark—the modern continuation of the Kramer sodium-borate ore body that displaced the old Death Valley colemanite camps—and it is a specimen locality whose best pieces can be unmistakable: snowy to colorless sprays of bladed colemanite on warm honey-orange calcite, satin-fibrous ulexite with a soft internal glow, borax and kernite masses from the sodium-borate core, and rare borates and arsenic sulfides that make the locality much more than an ore mine.
Geologically, Boron is the exposed industrial face of a buried Middle Miocene lacustrine system. The ore formed in a small nonmarine structural basin, where boron-rich thermal waters fed a persistent shallow lake. In the heart of the deposit are crystalline sodium borates—chiefly borax and kernite—with interbedded claystone. Around that core, the mineralization grades outward into ulexite-bearing and colemanite-bearing claystone and shale, then into barren claystone. That zonation is the key to understanding the specimens: the showy cabinet minerals favored by collectors did not come from one simple “borax bed,” but from distinct chemical and textural zones within and around a large, concealed evaporite body.
Historically, the locality begins with John K. Suckow’s 1913 water-well drilling, when borate nodules—not water—announced the deposit. Pacific Coast Borax explored and developed the ground after World War I; large-scale underground sodium-borate mining began in the 1920s, and the modern open pit followed in the late 1950s. Today the mine is operated by U.S. Borax, part of Rio Tinto, and remains one of the defining borate operations in the world.
Regional View
Country View
The best Boron specimens are more refined than the scale of the mine would suggest. Instead of giant ore chunks, serious collectors prize sharp, undamaged, glassy colemanite blades, balanced colemanite-calcite combinations, unusual golden to orange calcite habits, clean ulexite, and rare species with secure labels from the old Minette, Speckels, Gordon, or other California borate collections. Many fine pieces preserve the mine’s internal geography in their labels—Initial Pit, Extension 16, Extension 21S, Baker Mine, West Baker, Jenifer Mine—names that matter because they connect a specimen to a particular chapter in the pit’s development.
Photo: Wikimedia Commons
Photo: Wikimedia Commons
Search for specimens: View all specimens from Boron Open Pit, Boron, Kern County, USA
The Boron open pit works the Kramer borate deposit, a large, buried evaporite system in the northwestern Mojave Desert. The deposit is a sedimentary, lacustrine borate body rather than a vein or pegmatite locality: borates accumulated in a Miocene lake basin fed by sodium- and boron-rich thermal waters, then were modified during burial and diagenesis. The central ore zone is a lenticular crystalline mass of borax and kernite with claystone partings. This sodium-borate facies is enveloped by ulexite-bearing claystone and colemanite-bearing claystone, and the broader section includes clay shale, tuffaceous beds, sand, shale, and the Saddleback Basalt beneath the borate-bearing sequence.
The mineralogical zoning explains why Boron produces both industrial ore and collector specimens. Borax and kernite dominate the sodium-borate core and are the principal commercial minerals. Ulexite and colemanite occur in the surrounding Na-Ca and Ca-borate facies, especially in claystone and shale zones. Calcite occurs in and above the main ore beds and in vugs, and it is one of the great aesthetic partners of Boron colemanite. The deposit also contains a remarkable suite of accessory borates, arsenic minerals, sulfides, carbonates, sulfates, clays, and zeolites.
Mining began underground. After Suckow’s 1913 discovery, the claims were taken up by Pacific Coast Borax, whose exploratory work led to the recognition of the main sodium-borate ore body in the 1920s. The Baker Mine became the early large-scale operation, and underground workings eventually extended for many miles before the open pit began to consume the old mine ground. U.S. Borax shifted to open-pit mining in the late 1950s, and Rio Tinto’s U.S. Borax operation continues to mine, crush, convey, dissolve, crystallize, filter, dry, and ship refined borate products from the site.
For collectors today, the mine should be treated as a fully active industrial operation, not a public collecting locality. The Borax Visitor Center sits on the rim of the operation and provides a sanctioned overlook, historical displays, and interpretation of the mine, but access to the working pit and dumps is controlled by the operator. Modern specimen availability comes largely through old collections, mine-employee collections, authorized material, and retail channels rather than casual field collecting.
The most storied specimen material is tied to the era when knowledgeable mine people and visiting collectors could rescue fine pieces from active exposures. Labels naming Jim Minette are especially significant. Minette worked for U.S. Borax for decades, became mine manager, and built with his wife Dawn one of the best-known borate collections ever assembled. Documented Boron finds include colemanite from Extension 21S collected by Minette in 1980, colemanite-calcite combinations from Extension 16 and the Initial Pit, golden calcite from 1980s collecting, and the notable mid-1980s probertite occurrence that produced some of the finest probertite specimens from the locality.
Colemanite is the signature display mineral of the Boron open pit, even though the mine’s industrial fame rests chiefly on sodium borates. Good Boron colemanite occurs as colorless, white, grayish, smoky, or brown-tinged bladed crystals, commonly in sharp spear-like groups and sprays on calcite; older dealer descriptions record transparent lustrous crystals around half an inch, while modern documented pieces include cabinet specimens with glassy blades to about 1.6 cm and crystal groups with individual crystals approaching 47 mm. The most desirable pieces are not simply “more colemanite,” but well-composed examples with bright luster, undamaged terminations, transparency, and contrast—especially white or colorless blades rising from honey, tan, or orange calcite. Labels to the Initial Pit, Extension 16, Extension 21S, and old Minette-associated material add important locality precision, and specimens with realgar or pararealgar inclusions are much scarcer than ordinary colemanite-on-calcite combinations.
Calcite from Boron is a collector mineral in its own right, not merely a matrix. It occurs in many crystal forms in and above the main ore beds and in vugs, ranging from tan and beige stacked crystal clusters to rich honey, yellow, golden-orange, and botryoidal aggregates composed of tiny dogtooth crystals. The classic association is calcite with colemanite: rounded, stepped, or rhombic calcite crystals carrying sprays or crusts of white bladed colemanite, sometimes creating sculptural small-cabinet to cabinet specimens. Particularly desirable Boron calcites are strongly colored, translucent, and undamaged; fine golden-orange botryoidal pieces from the Jim Minette circle and rare near-complete calcite pseudomorphs or replacements after ulexite stand well above the more common pale tan calcite masses.
Ulexite at Boron belongs to the Na-Ca borate facies that envelopes the central borax-kernite ore, and it occurs as white to colorless fibrous, satin-like, compact, nodular, acicular, and locally pseudomorphic material. Clean compact blocks can show the fiber-optic “TV stone” character, while more mineralogically interesting pieces include sprays of acicular white ulexite on small bladed colemanite and ulexite after borax pseudomorphs preserving rounded prismatic borax forms with silky fibrous surfaces and light gray to brownish-orange clay or iron staining. Ordinary ulexite is soft, chalky, and massive; the best Boron specimens are complete, bright, satin-lustrous, clean, well-shaped, and securely localized, with associations such as colemanite, calcite, kurnakovite, tunellite, realgar, inderite, native arsenic, or probertite adding further significance.
Beyond colemanite, calcite, and ulexite, Boron is one of the great systematic borate localities. It is tied to type-locality or historically defining material for kernite, probertite, tunellite, and mazzite-Na, and the documented assemblage includes borax, tincalconite, inderite, kurnakovite, hydroboracite, inyoite, meyerhofferite, sassolite, searlesite, gerstleyite, realgar, pararealgar, orpiment, stibnite, native arsenic, rhodochrosite, dolomite, gypsum, halite, epsomite, and a sodium-rich zeolite suite in basalt cavities. For advanced collectors, Boron is therefore both a display locality and a reference locality: the aesthetic pieces are beautiful, but the rare borates and zeolites carry much of its deeper mineralogical weight.
Boron specimens require careful attention to label precision. “Boron,” “Kramer,” “Kramer District,” “U.S. Borax Mine,” “Rio Tinto Borax Mine,” “Boron open pit,” “Baker Mine,” and “West Baker” are not always used consistently in the specimen trade, and older labels may predate the Rio Tinto name. A good label should ideally preserve the older mine name and the modern geographic context rather than replacing one with the other. Specimens specifically from the active open pit should not be casually conflated with nearby Kramer district localities unless the provenance supports it.
The main authenticity concern is not widespread fakery but misidentification and locality drift. Ulexite, colemanite, probertite, inyoite, inderite, and other pale hydrous borates can look deceptively similar, especially when massive, fibrous, or partly dehydrated. Borax can alter to tincalconite; realgar can alter toward pararealgar; and yellow-orange arsenic sulfide material historically called orpiment may require analytical confirmation. Fine rare-borate specimens from Boron, especially probertite, tunellite, kurnakovite, inderite, and unusual ulexite forms, deserve X-ray, Raman, or provenance support when prices are serious.
Condition is a major factor. Colemanite is brittle and commonly cleaved or chipped at blade tips; sharp, glassy, complete terminations are worth a premium. Calcite can be bruised along exposed crystal edges, and sawed bases are common on older Boron display pieces because matrix was trimmed for stability. Ulexite is soft, fibrous, and easily abraded; it should be kept dry and handled minimally. Borax and tincalconite are water-soluble enough that cleaning mistakes can ruin them. Hydrous borates generally should not be washed, soaked, oiled, or stored in unstable humidity. Keep them away from direct heat, water, and prolonged harsh sunlight, particularly arsenic-bearing associations such as realgar, pararealgar, and orpiment.
Fluorescence can be present in Boron material, and borates from the district are sometimes represented in fluorescent collections, but response varies by species, impurities, and matrix. Do not buy a Boron specimen solely on a generic fluorescence claim without seeing the response under the wavelength you collect. Arsenic minerals, where present as inclusions or coatings, should be treated sensibly: avoid dust, do not lick or wet-test, wash hands after handling, and keep fragile material boxed.
Market availability is uneven. Colemanite on calcite remains obtainable, but top examples with saturated orange calcite, undamaged glassy colemanite, or old Minette/Speckels/Gordon provenance are much scarcer than the locality name suggests. Good calcite alone is rare in strong color and sculptural habit. Ulexite is widely known from Boron, but truly clean, aesthetic, well-localized specimen-grade pieces are not the same as common teaching pieces of “TV stone.” Rare borates and type-locality minerals appear sporadically, often from old collections; when documented and attractive, they are quickly absorbed by systematic collectors.
The Boron story begins with a wonderfully wrong first impression. In 1913, John K. Suckow was not opening a world-class borate mine; he was drilling for water in the Mojave. The drill brought up pale nodules that were first thought to be gypsum. Testing showed that the material was colemanite, and the accidental water-well discovery became the opening scene in the history of the Kramer borate deposit. The deposit did not crop out in the ordinary prospector’s way. Its wealth lay hidden beneath alluvium and lake-bed sediments, so the first clue came not from a gleaming vein on a hillside but from a drill hole in desert ground.
The 1920s brought the transformation from curious borate discovery to industrial turning point. Pacific Coast Borax explored after World War I, and the discovery of borax and kernite in 1925 changed the economic center of gravity for the company. In 1926 the Baker Mine went into large-scale underground sodium-borate production roughly two miles east of Suckow’s discovery well. The old calcium-borate districts of Death Valley had made borax famous with mule-team mythology, but Kramer offered a concealed, high-grade sodium-borate body of a different order. By the late 1950s, the underground era gave way to the open pit, and the old mine workings were gradually overtaken by benches, haul roads, crushers, and conveyors.
For specimen collectors, the great human figure is James W. “Jim” Minette. Born in Iowa, trained in mining engineering, and hired by U.S. Borax and Chemical Co. in 1959, Minette moved to Boron and became both an insider at the mine and one of the most consequential borate collectors in the world. He eventually worked his way from mine engineer to mine manager, and his position mattered: he understood what a borate pocket meant before it became crusher feed. Accounts of the Minette collection emphasize not just size but judgment—especially the famous thumbnail collection, each specimen fitting within a one-inch cube and chosen for exceptional quality.
One of the most repeated Boron collecting episodes concerns probertite. In 1985, Minette encountered a significant occurrence of well-formed probertite crystals in the pit. Recognizing its specimen importance, he redirected mining activity long enough to allow material to be collected. That detail is central to understanding why so many great Boron specimens carry mine-person provenance: in a working open pit, a world-class mineral occurrence can exist for only a brief interval between exposure and destruction. The best collector is not merely the person with access, but the person who knows when an exposure is scientifically and aesthetically worth saving.
The release of the Jim and Dawn Minette collection at Tucson in 2008 became a collecting event of its own. The Mineralogical Record’s show report described the sale as one of the major excitements of the Main Show, with specimens offered through Dave Bunk and Dan Weinrich. Before the doors opened, collectors formed charged lines at the dealers’ booths; when the show began, the waiting turned into a rush. The commerce continued for four days. That scene says something about Boron specimens that auction records alone cannot: by 2008, collectors understood that the best Minette Boron pieces were not replaceable mine products, but rescued moments from exposures that no longer existed.
Even the public overlook has its own theatrical quality. The Borax Visitor Center stands on the rim of the active mine, turning an industrial pit into a landscape-scale exhibit. One modern account describes the visitor-center video ending with curtains parting to reveal the operation below, the desert light flooding in and the mine stretching across the view. For a mineral collector, it is an unusual sensation: the locality is visible, immense, and close, yet the specimens themselves are mostly obtainable only through history—old labels, old mine people, old flats, and the occasional carefully documented release from collections.
