Carnelian (red chalcedony), possibly red agate or jasper

Polished/tumbled specimen with deep red to reddish-brown coloration, translucent orange-red edges, and irregular pale gray or cream inclusions. Vitreous to waxy luster; cryptocrystalline quartz structure; no visible cleavage; conchoidal fracture. Mohs hardness approximately 6.5–7; specific gravity about 2.58–2.65. The partly translucent edge favors carnelian or red chalcedony, while the opaque maroon areas may be jasper or iron-rich agate. Exact identification cannot be confirmed from an image alone.

Forms when silica-rich groundwater fills cavities and fractures in volcanic rocks, depositing microcrystalline quartz. Iron oxides and hydroxides produce the red, orange, and brown colors; heating can intensify the color. Related agate and jasper formed mainly in volcanic environments during many different periods, so there is no single geological age. The pictured piece has been cut and polished or tumbled.

Used for beads, cabochons, carvings, ornaments, and low-cost jewelry. Historically used for seals, signet rings, amulets, and decorative objects. It is also popular as a mineral-collection and lapidary material. It has little construction or industrial use compared with ordinary quartz.

Carnelian is often grouped with red chalcedony, although the boundaries between carnelian, sard, red jasper, and red agate are informal and overlapping. Ancient Egyptians, Greeks, Romans, and Islamic cultures used red chalcedony for seals and jewelry. Natural color is generally stable, but some commercial material is heat-treated or dyed.

In the field, carnelian is recognized by its orange-red to reddish-brown color, waxy or glassy polish, hardness sufficient to scratch ordinary glass, lack of cleavage, and translucent edges when held to a strong light. Red jasper is usually more uniformly opaque and granular-looking, whereas agate commonly shows curved banding. A knife should not scratch it easily. A streak, specific-gravity, microscopic, or spectroscopy test would help distinguish natural carnelian from dyed agate or glass. Common sources include India, Brazil, Uruguay, Madagascar, Botswana, Germany, and parts of the western United States.