Tube Agate Identification Guide
Identify tube agate by the hollow or filled mineral tubes running through translucent chalcedony, its hardness, and how it differs from sagenite and dendritic agate.
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What Tube Agate Looks Like
Tube agate is a chalcedony (microcrystalline quartz, SiO2) containing slender tube-like inclusions or channels that pass through the stone. The tubes form when needle-like mineral crystals (or growth structures) are enveloped by silica, sometimes leaving hollow straws and sometimes mineral-filled rods. When sliced across, the tubes appear as small circles or rings; sliced lengthwise, they look like straight or curving rods.
- Color: Translucent gray, white, bluish, or pale chalcedony base, often with the tubes in contrasting colors (white, red, brown, green).
- Luster: Waxy to vitreous.
- Transparency: Translucent to semi-transparent.
- Habit: Massive chalcedony with linear tube inclusions; commonly cut to display the tubes.
Field-ID Checklist
- Look for tube or rod inclusions running through the stone.
- Check both views — circles/rings on a cross-cut, lines on a lengthwise cut.
- Confirm translucency of the chalcedony base.
- Test hardness — 6.5–7; scratches glass.
- Apply acid — quartz does not fizz.
- Note conchoidal fracture on broken edges.
Key Diagnostic Tests
- Mohs hardness: 6.5–7. Scratches glass; not scratched by steel.
- Streak: White.
- Cleavage/fracture: No cleavage; conchoidal fracture.
- Acid test: Inert (no effervescence), ruling out carbonate.
- Density: ~2.6.
- Inclusions: Hollow or mineral-filled tubular channels are the diagnostic feature.
Common Look-Alikes
- Sagenite agate: Contains needle-like radiating mineral inclusions; tube agate's inclusions are tubular (often hollow) rather than fine solid needles, and tube agate frequently shows ring cross-sections.
- Dendritic/tree agate: Has branching plant-like dendrites, not straight tubes.
- Rutilated quartz: Transparent quartz with solid golden rutile needles; tube agate is translucent chalcedony with hollow or filled tubes.
- Moss agate: Irregular mossy inclusions rather than discrete tubes.
- Stinking water plume / plume agate: Feathery plumes, not linear tubes.
Where It's Found
Tube agate occurs in volcanic host rocks worldwide, often alongside other agate types. Well-known material comes from the United States (Oregon, Montana, and other western states), Mexico, Brazil, and India. It forms in gas cavities (vugs) of volcanic rock where silica-rich solutions precipitate chalcedony around pre-existing needle or tube structures.
Frequently asked questions
How can you tell if it's real tube agate?
Real tube agate is hard (Mohs 6.5–7) translucent chalcedony containing tubular inclusions or channels — visible as rods lengthwise and rings in cross-section. It does not fizz in acid and breaks with a conchoidal fracture.
What is the difference between tube agate and sagenite agate?
Tube agate contains tubular, often hollow channels that show as rings when cut across, while sagenite agate contains fine, solid, needle-like radiating mineral crystals.
What does tube agate look like?
It looks like translucent agate threaded with slender tubes or rods of contrasting color; cut one way you see straight lines, and cut the other way the tubes appear as little circles.
How are the tubes in tube agate formed?
The tubes form when silica-rich solutions surround pre-existing needle-like crystals or growth structures in a cavity, sometimes leaving hollow channels and sometimes mineral-filled rods after the original material dissolves or remains.
Tube Agate identified by the community
Recent Tube Agate specimens identified with Rock Identifier.
