W003 / Montana
Montana agate
Montana moss agate · a regional lapidary name
Hold a thin slice to the light and the little branches become easier to see. Montana agate is collected from river gravels; the river moved it, rather than making it.
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Specimen and photograph: Brooks Britt / Bhbritt54 · Released into the public domain by the author · No crop or color edits; scaled to fit. Exact provenance is not supplied.
Useful source-labelled comparison; exact locality is not independently authenticated. Visual resemblance does not identify C&C inventory.
What it is
Chalcedony cutting material, often translucent, with dark or warm-colored inclusions.
What to look for
Clear to cloudy areas may frame black dots, branching forms or brown-orange patches. Look in reflected and transmitted light. Those features are not exclusive to Montana.
At the cutting bench
Commonly sliced for cabochons or small display pieces. Examine how the pattern changes with thickness before cutting. Photograph dry, wet and backlit views separately; do not let a dramatic light treatment hide cracks or poor transparency.
Under the loupe
Compare inclusion shape, transparency and banding with documented specimens. Similar dendritic agates occur elsewhere. Visual agreement supports a comparison, not a guaranteed Montana locality.
Source & collecting access
UNKNOWN — no parcel-specific 2026 collecting authorization was verified. Montana FWP explains recreational stream access, but that is not blanket permission to remove rocks, enter private uplands or collect commercially.
Status checked September 2026The market notebook
Market data insufficient for a reliable range. DD Rocks slab223 asks US $14.95 for an unpolished, wet-photographed 3.75 × 2.25 inch slab, about 5.6 mm thick and 63 g. Copper Canyon slab2223 asks US $18.25; its generic dimensions conflict with the separately stated thickness. Both are asking prices.
Market researched 30 September 2026Open the field notes
Names, labels & aliases
Montana moss agate. “Moss” describes an inclusion pattern, not preserved moss. Yellowstone River agate is a geographic label whose precise collecting location still matters.
How it formed
Collectors find the agates in alluvial deposits—material carried and sorted by water—and in older river terraces. The original silica grew before the agates reached those gravels. A precise source rock for an individual loose pebble is not established here.
The collecting story
Rock & Gem’s collector account documents Yellowstone collecting and lapidary work. Montana’s geological bureau devoted its 2023 calendar to this material. Neither establishes a universal discovery date or the age of a particular dealer’s stock.
Reading an old-stock label
There is no verified old-stock-only variety in this record. Retain collector notes naming the river reach and collecting date. Weathering or polish alone cannot date the collection.
What we know—and what we don’t
WELL SUPPORTED: Yellowstone gravel occurrence and regional trade name. UNKNOWN: parcel access, individual bedrock origin and reliable price range. Full MBMG calendar retrieval remains incomplete.
C&C Reference Collection
No C&C reference specimen has been authenticated for this record yet. Acquired in Oregon is not the same as formed in Oregon. Preserve an estate’s original wording and mark any lost locality documentation.
Sources & Further Reading
- Montana Bureau of Mines and Geology · 2023 Montana Moss Agate calendar
Publication verified; indexed Yellowstone gravel occurrence checked. Full calendar PDF could not be retrieved in this audit.
- Rock & Gem · Montana Agate, Jim Landon (2019)
Collector reporting on river and terrace gravel; not parcel-level permission. Full-page retrieval later returned 403.
- University of Nebraska–Lincoln · Agates Collection
Historical names and locality-labelled reference specimens. Older entries are not current access advice; some geological interpretations need updating.
- DD Rocks · Montana slab223
Observed unpolished, wet-photographed slab asking price with dimensions and weight.
- Copper Canyon · Montana slab2223
Observed asking price. Thickness given separately; generic dimensions not treated as slab dimensions.
- Montana FWP · Stream Access
Explains recreational access. River access is not proof of a right to remove or sell material.
- CDC/NIOSH · Safe Work Practices for Silica
Dust control guidance; wet methods are one control, not a guarantee of safe exposure.
Silica-bearing stone can release hazardous dust when cut or ground. Use appropriate dust controls; wet working alone is not a safety guarantee.