Montana Agate is a variety of chalcedony renowned for its mossy or dendritic inclusions of manganese and iron oxides. Collectors typically find these stones as water-worn cobbles in the gravel beds of the Yellowstone River and its tributaries. They are highly prized by lapidary artists for their beautiful patterns when slabbed and polished.
Is this montana agate?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch montana agate with a known reference. Montana Agate sits at Mohs 6.5-7 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Montana Agate leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Montana Agate typically shows a waxy luster.
- 4Match the color rangeCompare against the expected color range: yellow, orange, brown, red, black, white, gray.
- 5Look at form & habitCrystal system: trigonal. Typical habit: massive.
Often confused with
Montana Agate vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside montana agate
Minerals reported to co-occur with montana agate. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SiO₂
- Mohs hardness
- 6.5-7
- Density
- 2.58-2.64 g/cm³
- Streak
- White
- Luster
- Waxy
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Lapidary, Collector, Decorative
- Host rock
- Alluvial Gravels and River Deposits
- Typical price
- $5-50 for rough, $20-200 for polished specimens
Where rockhounds find montana agate
7 mapped spotsClassic worldwide localities
- Yellowstone River, Montana, USA
- Missouri River, Montana, USA
- Powder River, Montana, USA
Field-hunting tip
Look in alluvial gravels and river deposits country — that is the host setting where montana agate typically forms. If you start seeing quartz, calcedony, iron oxides in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Montana — start trip planning there.
Common questions
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Find montana agate on the map
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