Chalcedony
A microcrystalline silica used in tools, gems, and art since prehistory.
Chalcedony is a mineral aggregate composed of cryptocrystalline or microcrystalline silica, meaning it consists of countless individual silica crystals too small to be see without magnification. Its primary component is quartz, but it also contains minor amounts of intergrown moganite, another silica mineral. While both share the same chemical composition, quartz crystallizes in the trigonal system, whereas moganite has a less symmetrical monoclinic structure. In strict mineralogical terms, the quartz and moganite crystals in chalcedony must grow in parallel, fibrous chains; however, the name is often applied more broadly to any microcrystalline or cryptocrystalline silica, fibrous or not. The stone exhibits a waxy luster and ranges from semitransparent to translucent. Its colors vary widely, with white, gray, grayish-blue, and shades of brown from pale to nearly black being most common, and commercial specimens are frequently enhanced through dyeing or heating. The name likely derives from the Latin *chalcedonius*, possibly linked to the town of Chalcedon in Asia Minor, though this connection is considered doubtful. Pliny the Elder used the term for a translucent kind of jaspis. A similar word appears in the Book of Revelation, but its meaning is uncertain. Chalcedony is more soluble than quartz under low temperatures, possibly due to its moganite content, defects from twinning, or small crystal size. Notable varieties include agate, carnelian, chrysoprase, heliotrope, jasper, moss agate, onyx, and fire agate, among others.
- composition
- SiO2 (quartz and moganite)
- crystal_system
- Trigonal (quartz) and monoclinic (moganite)
- luster
- Waxy
- transparency
- Semitransparent to translucent
- common_colors
- White to gray, grayish-blue, brown
- notable_varieties
- Agate, carnelian, chrysoprase, heliotrope, onyx, sardonyx
Lore & Background
Chalcedony is an aggregate of cryptocrystalline or microcrystalline silica, meaning it consists of a mass of individual silica crystals too small to be seen without magnification. It is composed primarily of quartz, with minor amounts of intergrown moganite; both are silica minerals with the same chemical composition, but quartz has a more symmetrical trigonal crystal structure, while moganite is monoclinic. Mineralogically, the quartz and moganite crystals in chalcedony must grow in parallel chains called fibers, though the term can also refer more broadly to any microcrystalline or cryptocrystalline silica, fibrous or not. The mineral has a waxy luster and can be semitransparent or translucent. It appears in a wide range of colors, most commonly white to gray, grayish-blue, or shades of brown from pale to nearly black; color in commercial chalcedony is often enhanced by dyeing or heating. Chalcedony is more soluble than quartz under low-temperature conditions, possibly due to its moganite component, defects from Brazil twinning, or its small crystal size. Notable varieties include agate (fibrous and banded), carnelian (reddish-brown), chrysoprase (green, colored by nickel oxide), fire agate (with iridescent inclusions of goethite or limonite), heliotrope or bloodstone (green with red iron oxide inclusions), moss agate (with green filament-like inclusions), and chrome chalcedony (green, colored by chromium compounds).
Reader's Guide
It was ground into powder for prehistoric cave paintings, as seen at the Bhimbetka rock shelters. In the Bronze Age, chalcedony seals were used on Minoan Crete, and later along Central Asian trade routes it was carved into intaglios and beads showing Greco-Roman influence. Its property of not sticking to hot wax made it ideal for seal impressions. The mineral's many varieties—including agate, carnelian, chrysoprase, heliotrope, and onyx—have been prized as semi-precious gemstones. According to tradition, at least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. The term 'plasma' is sometimes used for green translucent chalcedony. Commercially, chalcedony is often enhanced by dyeing or heating.
Did You Know?
- Chalcedony is composed primarily of quartz with minor amounts of moganite, a quartz polymorph that crystallizes in the monoclinic system.
- The name chalcedony may derive from the town of Chalcedon in Asia Minor, but the Online Etymology Dictionary calls that connection 'very doubtful'.
- Heliotrope, a green variety of chalcedony with red inclusions, is also known as bloodstone.
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