gemstone

Polished agate/chalcedony slab

Microcrystalline quartz (SiO₂), chiefly chalcedony with iron-oxide inclusions

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent white, gray, orange, red, and black zones; waxy to vitreous luster; cryptocrystalline, with no cleavage and conchoidal fracture. The irregular piece appears cut or polished, with botryoidal/color-banded textures.

Color: banded textures

Identified August 13, 2026More gemstone →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing chalcedony in successive layers; iron-bearing fluids create the orange-red coloration. Individual agate bands commonly formed during the Cenozoic or earlier, depending on the host volcanic rock.

Uses & applications

Used for cabochons, carvings, pendants, paperweights, and decorative slabs; durable and popular in lapidary work.

Geological facts

Agate is a variety of chalcedony, and its bands can record changing groundwater chemistry. Black areas may be manganese/iron oxides or darker included minerals.

Field identification & locations

Identify by hardness, waxy translucence, conchoidal fracture, and curved or mossy internal patterns; it will scratch glass but not normally be scratched by a steel knife. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the United States.