gemstone

Sardonyx / Carnelian Agate

SiO2 (Silicon Dioxide - Microcrystalline Quartz)

Hardness: 6.5-7 on Mohs scale; Color: reddish-brown, deep orange-brown with translucent banding; Luster: waxy to vitreous when polished; Crystal Structure: Trigonal (microcrystalline aggregate); Cleavage: none, conchoidal fracture; Specific Gravity: 2.58-2.64

Hardness: 6
Color: reddish-brown, deep orange-brown with translucent banding
Luster: waxy to vitreous when polished

Identified July 25, 2026More gemstone →

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Physical properties

Formation & geological history

Formed in cavities within volcanic and sedimentary rocks where silica-rich groundwater precipitates over long periods, creating concentric or parallel banding containing iron oxide impurities that produce the deep reddish-brown color.

Uses & applications

Popularly used for tumbled stones, decorative carvings, beads, lapidary art, and jewelry as a birthstone or healing crystal.

Geological facts

Sardonyx has been used since ancient Roman times, often carved into cameos or used in signet rings because hot wax does not stick to it.

Field identification & locations

Identified by its distinctive deep brownish-red to orange hue and subtle translucent banding when held up to light. Tumbled and polished pieces like this are common in rock shops, found in major deposits in India, Brazil, Madagascar, and Uruguay.

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