How to identify Tree Agate

To correctly identify Tree Agate (Chalcedony (variety of Quartz)), 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

   White, colorless, or milky translucent base with green, brown, or black dendritic (tree-like or fern-like) patterns.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to vitreous (glassy) when polished, dull to waxy on rough surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular, conchoidal fracture surfaces are smooth.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning individual crystals are microscopic and not visible to the naked eye. Forms botryoidal, mammillary, or stalactitic masses, or as vein fillings and nodules.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its cryptocrystalline nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in amygdaloidal cavities in volcanic rocks (e.g., basalt, andesite), hydrothermal veins, and as nodules in sedimentary deposits. Often associated with areas of past volcanic activity or silica-rich groundwater flow.

Key Facts

Hardness

6.5-7 on the Mohs scale.

Specific Gravity

2.58-2.64 g/cm³.

Crystal System

Trigonal (as a variety of Quartz, though cryptocrystalline).

Color

White, colorless, or milky translucent with green, brown, or black dendritic inclusions.

Luster

Waxy to vitreous.

Transparency

Translucent to opaque.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Silicon dioxide (SiO2) with inclusions of manganese oxides (e.g., MnO2) or iron oxides (e.g., Fe2O3).

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, typically massive, botryoidal, mammillary, or stalactitic. Individual crystals are microscopic.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal to uneven, splintery.
• Tenacity: Brittle.
• Luster Type: Waxy to vitreous (glassy).

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