A Lexicon of Tucson Rock and Mineral Terms: Vocabulary for the Southern Arizona Collector

Joel Bennett

Geolintix Consulting, Tucson, Arizona

Field Guide Series · No. 4 · Revised 2026

Keywords: Tucson, glossary, lexicon, mineralogy, rockhounding terminology, lapidary, geology vocabulary, Sonoran Desert

1. Scope and Arrangement

This lexicon defines the working vocabulary of rock and mineral collecting as practiced in the Tucson region. It is deliberately selective: an exhaustive mineralogical dictionary would serve the local collector less well than a compact reference restricted to terms actually in local use. Terms are drawn from five domains—mineral and rock names, structural and regional geology, desert geomorphology, land tenure, and the lapidary arts—reflecting the range of subjects a Tucson collector must navigate. Entries carrying a specifically regional significance are annotated as such within the definition. The lexicon is arranged alphabetically for reference use and is intended to be read alongside the companion papers of this series, whose subjects it supports.

Agate n.

A translucent, commonly banded variety of cryptocrystalline quartz. A staple collecting and tumbling material in the Tucson-area washes; distinguished from jasper by its transmission of light at the edges.

Alluvium n.

Unconsolidated sediment—sand, gravel, silt, clay—deposited by flowing water. Regional: the entire floor of the Tucson Basin is alluvium eroded from the surrounding ranges; it is the material through which the washes cut and in which water-transported specimens are concentrated.

Apache tears n.

Rounded nodules of obsidian that appear opaque in reflected light but transmit smoky brown light when held to a bright source. Weather out of a perlitic volcanic matrix; a common and easily identified Arizona collectible.

Azurite n.

A deep-blue copper carbonate mineral of the oxidized zone of copper deposits, frequently associated with green malachite. A characteristic mineral of southern Arizona copper country.

Basin and Range n.

The physiographic and structural province, produced by crustal extension, in which Tucson lies. Regional: responsible for the alternating uplifted ranges and sediment-filled valleys of the Tucson landscape, and thus for the exposure of varied rock close to the city.

Botryoidal adj.

Having a globular, grape-cluster surface form. Descriptive of many chalcedony and secondary-copper specimens found in the region.

Cabochon (cab) n.

A gemstone cut with a smooth, domed upper surface and a flat base, as opposed to a faceted cut. The standard form for turquoise, chrysocolla, and agate in Southwestern lapidary work.

Caliche n.

A near-surface soil horizon cemented by calcium carbonate, ranging from chalky to concrete-hard. Regional: pervasive in the Tucson subsurface; impedes drainage, complicates excavation, and functions as a "false bedrock" that traps heavy minerals in washes.

Chalcedony n.

A compact, typically translucent form of cryptocrystalline quartz with a waxy luster. Common in the volcanic terrain west of Tucson, frequently in botryoidal form.

Chrysocolla n.

A hydrated copper silicate of striking blue to blue-green color. Regional: among the signature colorful minerals of Tucson's copper country; the quartz-hardened "gem silica" grade is a valuable lapidary material.

Cryptocrystalline adj.

Composed of crystals too small to distinguish without magnification. Agate, jasper, and chalcedony are cryptocrystalline forms of quartz.

Crinoid n.

A stalked marine echinoderm ("sea lily") whose disc-shaped stem segments are common fossils. Regional: the most frequently encountered fossil in the Paleozoic limestones flanking the ranges around Tucson.

Druzy n./adj.

A surface coating of minute, sparkling crystals lining a cavity or covering a substrate. Valued in jewelry for its glitter.

Extension n.

The tectonic stretching of the crust responsible for the Basin and Range province. Regional: the fundamental process that fragmented, uplifted, and exposed the varied rock of the Tucson ranges.

Fire agate n.

A brown chalcedony containing ordered thin films of iron oxide that yield an iridescent internal play of color when precisely ground. Regional: a celebrated southern Arizona specialty, demanding to cut.

Float n.

Loose rock or mineral fragments displaced downslope or downstream from their bedrock source. Tracing float toward its origin is a basic prospecting technique.

Fusion crust n.

The thin, dark, melted rind on a freshly fallen meteorite. Relevant to the regional distinction between meteorites and the far more common "meteorwrongs."

Gneiss n.

A coarsely banded metamorphic rock formed under high temperature and pressure. Regional: Catalina gneiss forms much of the Santa Catalina Mountains and is well exposed along the Catalina Highway.

Hardness n.

Resistance to scratching, expressed on the Mohs scale of 1–10. Quartz is 7; the ability to scratch glass (about 5.5) is a routine field test.

Hydrothermal adj.

Pertaining to hot, mineral-bearing water. Regional: hydrothermal circulation associated with porphyry systems produced the copper mineralization of southern Arizona.

Jasper n.

An opaque, solidly colored cryptocrystalline quartz, typically red, yellow, or brown from iron oxides. Common Tucson-area tumbling material.

Lapidary n./adj.

The art of cutting, grinding, and polishing stone; a practitioner thereof. Regional: Tucson's clubs and exhibition make it a leading center for learning the craft.

Malachite n.

A green, commonly banded copper carbonate of the oxidized copper zone, frequently paired with azurite. A staple of Arizona copper districts.

Matrix n.

The host rock enclosing a mineral or gem. "Turquoise in matrix" retains veins of host rock, often regarded as desirable.

Metamorphic adj.

Formed by the alteration of pre-existing rock under heat and pressure. Regional: the Santa Catalina and Rincon Mountains are dominantly metamorphic.

Mining claim n.

A legal location conferring rights to locatable minerals on public land. Regional: critical to lawful collecting—a claim can encumber otherwise-collectible BLM or Forest Service land, rendering it off-limits without the claimant's permission.

Monsoon n.

The summer convective storm season (roughly July–September). Regional: its floods scour and restock the Tucson-area washes annually, renewing the collector's supply—and posing a flash-flood hazard.

Obsidian n.

Volcanic glass formed by rapid cooling of lava. Occurs regionally chiefly as Apache tears.

Oxidized zone n.

The weathered upper portion of a mineral deposit where primary minerals are altered by air and groundwater. Regional: the source of the colorful secondary copper minerals (chrysocolla, malachite, azurite, turquoise).

Peridot n.

Gem-quality olivine of volcanic origin. Regional: Arizona is a world source; ubiquitous at the Tucson exhibitions though the chief deposit is not open to public collecting.

Porphyry copper n.

A large, low-grade, disseminated copper deposit associated with intrusive magma. Regional: the deposit type responsible for southern Arizona's copper wealth and its collectible copper minerals.

Quartz n.

Crystalline silicon dioxide, hardness 7; the most abundant collectible mineral around Tucson, in forms from clear crystal to milky vein, smoky, and amethyst.

Rockhounding n.

The amateur pursuit of rocks, minerals, and fossils. Regional: a defining regional pastime, supported by the geology, the desert exposure, and the exhibition culture.

Slab n.

A flat slice sawn from rough rock, ready to be marked and cut into cabochons. Purchasing slabs allows lapidary practice before acquiring cutting machinery.

Stabilized adj.

Of turquoise or other porous stone, impregnated with resin to permit cutting and wear. Regional: most turquoise sold at the Tucson exhibitions is stabilized—legitimate when disclosed and priced accordingly.

State trust land n.

Arizona land held in trust for designated beneficiaries. Regional: extensive around Tucson and not open public land; requires a permit and does not authorize casual collecting.

Streak n.

The color of a mineral's powder, obtained by drawing it across unglazed tile; often more diagnostic than surface color and decisive in identifying the copper minerals and separating meteorites from meteorwrongs.

Subsidence n.

The sinking of the ground surface, in this region caused chiefly by groundwater withdrawal. Regional: associated with the earth fissures of certain southern Arizona basins.

Turquoise n.

A blue-to-green hydrated copper aluminum phosphate; Arizona's signature stone. Regional: forms in the region's copper country and is widely imitated by dyed howlite and confused with chrysocolla.

Tuff n.

A rock formed of consolidated volcanic ash. Regional: ash-flow tuffs are prominent in the Tucson Mountains west of the city.

Vug n.

A small rock cavity, often crystal-lined. Opening a vug to reveal a crystal pocket is among the collector's small rewards.

Wash n.

A normally dry desert watercourse that flows during storms. Regional: the primary collecting ground around Tucson, concentrating water-worn material and renewed each monsoon.