Best field clue: Translucent chalcedony with dendritic or moss-like inclusions.

Montana moss agate is a chalcedony variety strongly associated with Yellowstone River gravels. Look for dense silica, translucent edges or windows, waxy-to-glassy luster, and branching or moss-like inclusions inside the stone. River rounding and a weathered rind are common, so wetting an already loose specimen can reveal internal pattern without damaging it.

Common look-alikes: Jasper, iron-stained quartz, slag.

Best field clue: Opaque, very fine-grained silica with smooth fracture.

Jasper is opaque microcrystalline silica. It can be red, yellow, brown, green, cream, or multicolored, so color alone is weak evidence. A strong field ID combines high hardness, very fine grain, and smooth to conchoidal fracture. Jasper and agate can grade into one another in the same specimen.

Common look-alikes: Agate, rhyolite, iron-stained rock.

Best field clue: Dense microcrystalline quartz, commonly translucent at thin edges.

Chalcedony is microcrystalline quartz and includes agate varieties. It is usually dense, smooth, and harder than a steel knife. Thin edges may transmit light even when the center looks cloudy. Banding is not required; unbanded chalcedony is common.

Common look-alikes: Quartz, opal, glass.

Best field clue: Hard mineral with no cleavage; hexagonal crystals when well formed.

Quartz is one of Montana’s most useful reference minerals. It is hard enough to scratch ordinary glass, lacks cleavage, and commonly breaks with conchoidal fracture. Well-formed crystals show six-sided prisms and pointed terminations, but massive white quartz may have no obvious crystal shape.

Common look-alikes: Calcite, feldspar, glass.

Best field clue: Gray to brown quartz that retains normal quartz hardness and fracture.

Smoky quartz is a color variety of quartz, so the reliable clues are quartz properties rather than darkness: hardness 7, vitreous luster, no cleavage, and conchoidal fracture. Transparent to translucent gray-brown color strengthens the ID when those properties agree.

Common look-alikes: Dark glass, smoky calcite.

Best field clue: Purple quartz with normal quartz hardness and crystal habit.

Amethyst is purple quartz. The color can be pale or uneven, so confirm quartz hardness, lack of cleavage, and crystal form where visible. Fluorite is much softer and has strong cleavage; purple glass may show bubbles or molded surfaces.

Common look-alikes: Fluorite, purple glass.

Best field clue: Extremely hard corundum; rounded grains can occur in placer gravel.

Montana sapphire can be blue, green, gray, yellow, or other colors. Color is not enough. Corundum hardness is the standout property, but potentially valuable stones should not be subjected to destructive scratch tests. Shape, density, locality, and professional confirmation matter.

Common look-alikes: Quartz, glass, spinel.

Best field clue: Equant crystals, often dodecahedral, with no cleavage.

Montana garnets are commonly red to brown and may occur as rounded grains or equant crystals. Look for a glassy luster, high hardness, lack of cleavage, and crystal faces when preserved. Deep red color by itself is not diagnostic.

Common look-alikes: Red glass, spinel, iron-rich pebbles.

Best field clue: Soft carbonate with strong rhombohedral cleavage.

Calcite is much softer than quartz and commonly breaks along three directions of rhombohedral cleavage. White or clear color is common but not diagnostic. Where testing is lawful and appropriate, hardness is a simple separator: calcite can be scratched by a steel knife, quartz cannot.

Common look-alikes: Quartz, gypsum, feldspar.

Best field clue: Wood grain, growth structure, or bark texture preserved in mineral replacement.

Petrified wood should preserve convincing biological architecture—grain, growth rings, cell-like texture, knots, or bark patterns—not merely brown striping. Silicified pieces can be as hard as quartz. Fossil and land-management rules still control whether material can be collected.

Common look-alikes: Banded jasper, sedimentary rock.

Best field clue: Brassy metallic cubes or striated crystal faces; brittle, not malleable.

Pyrite is brass-yellow rather than rich gold-yellow and commonly forms cubes or other sharply faced crystals. It is harder and more brittle than native gold. A sparkling yellow mineral that crushes, splinters, or shows cubic form is behaving more like pyrite than gold.

Common look-alikes: Gold, chalcopyrite, mica.

Best field clue: Unusually heavy for its size; tabular or blocky crystals.

Barite’s most memorable field clue is heft. Its specific gravity is high, so a modest specimen can feel unexpectedly heavy. It is relatively soft and commonly occurs in tabular or blocky forms. Heft is useful only when combined with hardness and crystal habit.

Common look-alikes: Calcite, gypsum, quartz.

Best field clue: Pistachio to yellow-green mineral, commonly prismatic or granular.

Epidote is commonly pistachio green to yellow-green and may be granular or prismatic. Color can attract attention, but hardness, crystal form, and host-rock context are more useful than green color alone.

Common look-alikes: Olivine, green tourmaline, altered rock.

Best field clue: Six-sided prismatic crystals; aquamarine is blue to blue-green beryl.

Beryl forms hexagonal prisms and is harder than quartz. Aquamarine is the blue to blue-green gem variety. Pale color can be subtle, so crystal form and hardness carry more weight than saturation. Valuable crystals deserve non-destructive confirmation.

Common look-alikes: Quartz, topaz, glass.

Best field clue: Dense pale-to-brown mineral; many specimens fluoresce under shortwave UV.

Scheelite is a tungsten mineral that can be white, tan, yellowish, orange, or brown and is denser than its appearance suggests. UV fluorescence can be a useful supporting clue for some specimens, but fluorescence alone is not a complete identification.

Common look-alikes: Quartz, calcite, barite.

Best field clue: Rich yellow, very dense, soft, and malleable rather than brittle.

Native gold is heavy, soft, and malleable. Small flakes bend or flatten rather than shatter. Pyrite is harder, brassier, and brittle. Locality and placer context help, but yellow color or sparkle alone should never be treated as proof.

Common look-alikes: Pyrite, chalcopyrite, mica.

Best field clue: Very hard rock of fused quartz grains; fracture tends to cross grains.

Quartzite is metamorphosed sandstone in which quartz grains have become tightly fused. It is very hard, and a fresh surface often shows a sugary or interlocking quartz texture. Unlike ordinary sandstone, it tends to break across grains rather than cleanly around them.

Common look-alikes: Quartz vein, sandstone, marble.

Best field clue: Very fine-grained, compact metamorphosed mudrock.

Argillite is a compact, very fine-grained metamorphosed mudrock. It is tougher and less fissile than shale but generally lacks the strong slaty cleavage of slate. Fresh surfaces are typically dull and may show subtle bedding or color bands.

Common look-alikes: Shale, slate, siltstone.

Best field clue: Visible sand-sized grains; often gritty and bedded.

Sandstone is a sedimentary rock made mainly of sand-sized grains. A hand lens may reveal individual quartz or feldspar grains and cement between them. Bedding, cross-bedding, and a gritty texture are common. Quartzite is usually harder and more tightly fused.

Common look-alikes: Quartzite, siltstone, volcanic tuff.

Best field clue: Fine to fossiliferous carbonate rock; commonly pale gray and relatively soft.

Limestone is widespread in Montana and commonly forms pale gray cliffs and ridges. It may preserve fossils or show fine carbonate texture. Calcite-rich limestone is relatively soft compared with quartz-rich rocks. Do not use acid on protected or in-place rock; field setting and texture often provide enough context.

Common look-alikes: Dolostone, marble, quartzite.

Best field clue: Very fine sedimentary layers; shale splits into thin sheets more readily.

Shale and siltstone are fine-grained sedimentary rocks common in layered basins. Shale is notably fissile and tends to split along thin bedding planes; siltstone feels slightly grittier and breaks more blockily. Metamorphism can transform similar material into argillite or slate.

Common look-alikes: Argillite, slate, mudstone.

Best field clue: Visible interlocking quartz and feldspar with mica or dark minerals.

Granitic rock is coarse enough that interlocking mineral grains are usually visible: light feldspar, glassy quartz, and dark mica or amphibole. Grain size and interlocking texture are stronger clues than overall pink, gray, or white color.

Common look-alikes: Diorite, rhyolite, quartzite.

Best field clue: Dark to medium-gray fine-grained volcanic rock; vesicles or small crystals may occur.

Basalt and andesite are volcanic rocks whose crystals are usually too small to identify easily by eye. Basalt is generally darker and more mafic; andesite is commonly medium gray. Vesicles, flow texture, and volcanic setting can support the identification, but slag can imitate both.

Common look-alikes: Dark limestone, slag, fine-grained intrusive rock.

Best field clue: Dark coarse intrusive rock with abundant augite and potassium feldspar; a Montana type locality name.

Shonkinite is an unusual alkaline intrusive rock famously associated with central Montana. In hand specimen it is typically dark and crystalline, commonly with conspicuous pyroxene and feldspar. Because precise classification can require petrographic work, use “shonkinite” cautiously outside well-established geologic context.

Common look-alikes: Dark syenite, gabbro-like rock, altered intrusive rock.