sedimentary
Agate (Beach Pebble)
Silicon Dioxide (SiO2) - Banded Chalcedony
Hardness of 7 on Mohs scale, vitreous to waxy luster, translucent to opaque with distinct translucent quartz banding and iron-oxide staining, trigonal crystal system (microcrystalline), no cleavage, specific gravity ~2.6.
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Physical properties
Hardness of 7 on Mohs scale, vitreous to waxy luster, translucent to opaque with distinct translucent quartz banding and iron-oxide staining, trigonal crystal system (microcrystalline), no cleavage, specific gravity ~2.6.
Formation & geological history
Formed in cavities within volcanic rocks or sedimentary formations as silica-rich groundwater deposited successive layers of chalcedony. Over time, erosion weathered the host rock, leaving the durable agate pebble to be rounded by wave or river action.
Uses & applications
Used in lapidary work, jewelry making, tumbling, ornamental decor, and as popular beachcombing collectibles.
Geological facts
Agates acquire their distinctive reddish-brown hues from iron oxide impurities trapped between microcrystalline quartz layers during deposition.
Field identification & locations
Identify in the field by checking for translucency under light, a smooth, polished water-worn surface, and distinct concentric or parallel banding pattern. Commonly found along beaches, riverbeds, and gravel bars globally.
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