Agate (Sliced Nodule)

Physical Properties

Hardness: 6.5–7 on Mohs scale. Color: Banded layers of brown, orange, white, and translucent gray. Luster: Waxy to vitreous. Crystal system: Trigonal (Microcrystalline). Specific gravity: 2.58–2.64.

Formation & Geological History

Formed in cavities of volcanic rocks or ancient lavas (geodes). Silica-rich groundwater percolates into these voids, depositing layers of microcrystalline quartz over thousands of years. Age varies by locale, often Mesozoic or Cenozoic.

Uses & Applications

Used primarily in lapidary arts for jewelry, decorative slices, bookends, and wind chimes. Industrially used for precision pendulums and laboratory mortars/pestles due to its hardness and chemical resistance.

Geological Facts

The banding in agates is often compared to tree rings, and no two specimens are exactly alike. Famous for the 'iris' effect in some thin slices where light diffracts into rainbow colors. Historically used as amulets for protection.

Field Identification & Locations

Identify in the field by looking for waxy, translucent stones with a conchoidal fracture. Commonly found in Brazil, Uruguay, Lake Superior (USA), and Germany. Collectors look for high contrast in banding and lack of cracks.

Identified on: 4/13/2026

Mode: Standard