How to identify Chalcedony

To correctly identify Chalcedony (SiO₂), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable; commonly white, gray, blue, brown, red, orange, green. Often banded or mottled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular, often with a botryoidal (grape-like) or mammillary (breast-like) habit.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Often forms botryoidal, mammillary, stalactitic, or nodular masses, or as vein fillings and amygdules.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits a conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in amygdaloidal cavities in volcanic rocks (e.g., basalt, rhyolite), as vein fillings, in geodes, as concretions in sedimentary rocks (e.g., limestone, shale), and as a replacement mineral in petrified wood.

Key Facts

Hardness

6.5 - 7 (Mohs scale)

Specific Gravity

2.55 - 2.65

Crystal System

Trigonal (cryptocrystalline aggregates of quartz and moganite)

Color

White, gray, blue, brown, red, orange, green; often banded or mottled

Luster

Waxy to dull, sometimes vitreous

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Silicon dioxide (SiO₂)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, typically botryoidal, mammillary, stalactitic, nodular, or as vein fillings. Individual crystals are microscopic.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Waxy to dull, sometimes vitreous

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