Priday Plume Agate Identification Guide
How to recognize Oregon's Priday plume agate by its three-dimensional feathery plumes inside thunderegg chalcedony.
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What Priday Plume Agate Looks Like
Priday Plume Agate is a plume agate from the famous Priday/Richardson Ranch beds of central Oregon. It is chalcedony (microcrystalline quartz) hosting delicate, three-dimensional plume inclusions of iron and manganese oxides that look like feathers, sprays, or puffs of smoke suspended in the stone. Plumes are commonly red, gold-yellow, black, or brown set in a milky-white, gray, or faintly bluish translucent base.
The material is found as thundereggs — rounded nodules weathered out of rhyolite — so rough pieces often look like knobby brown balls until cut.
Step-by-Step Field ID Checklist
- Form: Look for rounded nodular thundereggs in or below rhyolite/tuff beds.
- Cut or wet a face: The interior reveals translucent chalcedony with feathery plumes.
- Plume geometry: Confirm the plumes are 3-D and feathery (they branch into the stone), not flat.
- Hardness test: It should scratch glass easily (Mohs 7).
- Luster/fracture: Note the waxy luster and conchoidal fracture typical of chalcedony.
Key Diagnostic Tests
- Mohs hardness: 7 — scratches glass and is not scratched by a steel knife.
- Streak: white.
- Cleavage: none; conchoidal fracture with sharp edges.
- Acid: no reaction (silica).
- Translucency: edges glow when backlit, confirming chalcedony rather than opaque jasper.
Common Look-Alikes and How to Tell Them Apart
- Dendritic agate: dendrites are flat, two-dimensional fern-like growths confined to a plane; Priday plumes are three-dimensional and feathery.
- Moss agate: contains tangled, random mossy filaments rather than organized feather-like plumes.
- Other plume agates (Graveyard Point, McDermitt, Sagenite): distinguished by locality, color palette, and base color; Priday material typically has a milky-blue to white base with red/gold/black plumes.
- Jasper with inclusions: jasper is opaque and does not transmit light at thin edges, unlike translucent plume agate.
The combination of translucent chalcedony, hardness 7, conchoidal fracture, three-dimensional feathery plumes, and thunderegg form points to plume agate.
Where It Is Found
Priday Plume Agate comes from the Priday Ranch / Richardson's Rock Ranch thunderegg beds in Jefferson County, central Oregon (near Madras), weathering from rhyolitic ash-flow tuffs. It is one of Oregon's classic rockhounding materials.
Frequently asked questions
What makes Priday plume agate different from dendritic agate?
Priday plumes are three-dimensional, feathery inclusions that branch into the stone, while dendrites are flat, two-dimensional fern-like patterns that lie along a single plane.
How can you tell if it's real Priday plume agate?
Look for translucent chalcedony with feathery 3-D plumes (red, gold, or black), a hardness of 7, conchoidal fracture, and an origin in Oregon thundereggs from the Priday/Richardson beds.
What does Priday plume agate look like?
It is a translucent milky-white to bluish chalcedony filled with delicate feather-like sprays of red, gold, or black mineral oxides, usually cut from rounded thunderegg nodules.
Where is Priday plume agate found?
It is collected from the Priday Ranch and Richardson's Rock Ranch thunderegg beds in Jefferson County, central Oregon, near Madras.