Limb casts are natural molds formed when molten volcanic debris buries a tree limb and the wood eventually rots away, leaving a hollow cavity that is later filled by silica-rich minerals. Collectors often find these as tubular or bark-textured rocks that can be broken open to reveal druzy quartz or colorful chalcedony linings. They are highly prized for their unique internal textures which preserve the shape of the original ancient vegetation.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

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Limb Cast

Hardness: 7
Luster: Vitreous
Streak: White
Transparency: Translucent

No image

Is this limb cast?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch limb cast with a known reference. Limb Cast sits at Mohs 7 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Limb Cast leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Limb Cast typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: clear, white, gray, brown, red, yellow.
5
Look at form & habit
Crystal system: trigonal. Typical habit: botryoidal.

Often confused with

Limb Cast vs. its common look-alikes — and how to tell them apart in the field.

How to tell apart: Luster reads vitreous on Limb Cast and waxy on Agate.

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Often found alongside limb cast

Minerals reported to co-occur with limb cast. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: SiO₂
Mohs hardness: 7
Density: 2.6-2.7 g/cm³
Colors: Clear White Gray Brown Red Yellow
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Trigonal
Crystal habit: Botryoidal
Cleavage: None
Rarity: Common
Uses: Lapidary, Collector, Decorative
Host rock: Volcanic Ash Beds
Typical price: $10-200 depending on size and internal crystallization

Where rockhounds find limb cast

Classic worldwide localities

Oregon, USA
Washington, USA
Idaho, USA
Patagonia, Argentina

Field-hunting tip

Look in volcanic ash beds country — that is the host setting where limb cast typically forms. If you start seeing chalcedony, quartz, opal in float, you are in the right ground. Field specimens usually show a botryoidal habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify limb cast?+

Mohs hardness is 7. It typically shows a vitreous luster. The streak is white. Common colors include clear, white, gray, brown.

Where is limb cast found?+

Notable localities include Oregon, USA; Washington, USA; Idaho, USA; Patagonia, Argentina.

How much is limb cast worth?+

Typical asking prices fall in the range of $10-200 depending on size and internal crystallization. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like limb cast?+

Limb Cast is most often confused with Geode, Agate. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with limb cast?+

Limb Cast commonly co-occurs with Chalcedony, Quartz, Opal. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does limb cast form in?+

Limb Cast typically forms in volcanic ash beds. Working float back to the host body is the standard way to chase a fresh occurrence.

What is limb cast used for?+

Limb Cast is used in lapidary, collector, decorative.

Find limb cast on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.