How to identify Chalcedony

To correctly identify Chalcedony (Chalcedony), 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

   Extremely variable; commonly white, gray, blue-gray, brown, black, red, orange, green, yellow. Often translucent with a milky or cloudy appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous (glassy) on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular, often botryoidal, mammillary, or stalactitic forms. Microcrystalline structure is not visible to the naked eye.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically massive, botryoidal, mammillary, stalactitic, or as nodular concretions. Individual crystals are microscopic and not discernible without high magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits a characteristic conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in amygdules and veins in volcanic rocks (e.g., basalt, rhyolite), as nodules and concretions in sedimentary rocks (e.g., limestone, shale), and in hydrothermal veins. Often associated with hot spring deposits and geysers.

Key Facts

Hardness

6.5-7 on the Mohs scale

Specific Gravity

2.55-2.65

Crystal System

Trigonal (due to quartz component), but cryptocrystalline aggregate

Color

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

Luster

Waxy to dull, sometimes vitreous

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Silicon dioxide (SiO2), often with trace impurities

Physical characteristics

• Crystal Habit: Cryptocrystalline, typically massive, botryoidal, mammillary, stalactitic, nodular, or as vein fillings.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven, very tough.
• Tenacity: Brittle to tough
• Luster Type: Waxy to dull, sometimes vitreous on fresh fracture.

Explore Chalcedony