Polyhedroid Agate Identification Guide
A field guide to polyhedroid agate, identifying its flat-sided geometric shape, internal banding, and chalcedony composition, and how it differs from ordinary nodular agate.
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What Polyhedroid Agate Looks Like
Polyhedroid agate is a rare chalcedony agate that forms in distinctive flat-faced, angular, polygon-shaped blocks rather than the usual rounded nodules. Externally it shows straight edges and flat faces (triangular, rhomboid, or many-sided geometric forms), as though the agate grew in a boxy mold. Inside, it reveals concentric to angular banding in grays, whites, browns, reds, and translucent blues. Luster is waxy to vitreous, and the body is translucent on thin edges.
Step-by-Step Field ID Checklist
- Look at the external shape. The defining trait is flat faces meeting at sharp angles, producing a polyhedral (many-flat-sided) form.
- Cut or examine a broken face. Internal banding should follow the angular outline, often with a chalcedony center.
- Confirm chalcedony. Waxy luster, conchoidal fracture, translucency on edges.
- Test hardness. Scratches glass easily.
- Check the source clue. Nearly all genuine polyhedroid agate comes from one region in Brazil.
Key Diagnostic Tests
- Mohs hardness: 6.5 to 7 — scratches glass and steel.
- Cleavage: None; conchoidal fracture.
- Streak: White.
- Specific gravity: About 2.6.
- Acid: No reaction to dilute HCl.
- Form: The flat-faced polyhedral external geometry is the single best diagnostic.
Common Look-Alikes and How to Tell Them Apart
- Ordinary nodular agate: Rounded, lumpy exterior versus polyhedroid's flat geometric faces.
- Polyhedroid quartz crystals: Polyhedroid agate is microcrystalline chalcedony (no visible crystal points), whereas quartz shows clear hexagonal crystal faces.
- Fortification agate: Has angular internal banding but typically a rounded nodule shape; polyhedroid is angular both inside and out.
- Cut/faceted agate (man-made shape): Faceting leaves polished tool marks and unnaturally regular geometry; natural polyhedroid faces are slightly irregular and matte where unpolished.
Where Polyhedroid Agate Is Found
Polyhedroid agate is almost exclusively found near Paraíba, Brazil. Its unusual shape is thought to result from agate filling polygonal gaps between other crystals (possibly former calcite or zeolite crystals that later dissolved), leaving flat-walled molds.
Frequently asked questions
How can you tell if it's real polyhedroid agate?
Look for a naturally flat-faced, angular polygonal shape with internal agate banding, waxy chalcedony texture, hardness near 7, and conchoidal fracture. Genuine pieces have slightly irregular natural faces, not machine-cut facets.
What does polyhedroid agate look like?
It looks like a many-sided geometric block of agate with flat faces and sharp edges, banded inside with grays, browns, reds, and translucent chalcedony.
Why is polyhedroid agate shaped like a polygon?
Its flat faces are thought to form when agate fills the angular gaps between other crystals that later dissolved, leaving polygonal molds that the chalcedony preserved.
Where does polyhedroid agate come from?
Almost all genuine polyhedroid agate comes from the Paraiba region of Brazil, making locality a strong identification clue.
Polyhedroid Agate identified by the community
Recent Polyhedroid Agate specimens identified with Rock Identifier.
